Moist & Tender Marble Cake

I made this super moist & delicious marble cake more than a week ago... had wanted to complete & publish this post much earlier but it had taken me a few logins before I could complete this. Anyway, I stumbled upon an awesome instagram by a blogger, Tasbih of Cleobuttera (I love the name too!) & couldn't wait to bake the marble cake that she had baked. Definitely one of the best marble cakes I've ever eaten before! Just look at the pictures below or just hop over to her blog post for more beautiful pictures of the cake. Don't you want to bake it too?

Moist & Tender Marble Cake

Ingredients:
2½ cups (8.7oz/ 250g) cake flour  (* see note below for the substitute)
2½ teaspoons baking powder - I used double-acting baking powder.
½ teaspoon baking soda
½ teaspoon salt
¾ cup (6.4oz, 181g) sour cream, at room temperature
¼ cup whole milk, at room temperature
1 cup (8oz, 227g) unsalted butter, softened to room temperature
1½ cups (10½oz, 300g) granulated sugar
2 large eggs, at room temperature
1 large egg yolk, at room temperature
1 tablespoon pure vanilla extract (1½ tablespoons if using imitation vanilla extract)

Method:

1. Adjust the oven rack to middle position and preheat the oven to 350F/180C.
2. Butter and flour (or generously spray with a non-stick cooking spray) a 10-cup bundt pan (*see note no. 2).
3. In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda and salt. Set aside. In a liquid measuring cup (or a small bowl), whisk together the sour cream and milk. Set aside.
4. In the bowl of a electric mixer fitted with a paddle attachment (or in a large bowl and an electric hand mixer), beat the butter on medium speed until creamy, ribbon-like and slightly lightened up; 2 to 3 minutes.
5. With the mixer running on medium-low speed, gradually add in the sugar. Raise the speed to medium-high and continue beating together until lightened up in both colour and texture; 4 to 5 minutes. The colour should turn very pale (almost white) and texture should look fluffy.
6. Add the eggs and egg yolk, one at a time, beating well after each addition. Add in the vanilla and beat together for one minute.
7. On lowest speed, add in one-third of the flour mixture and mix until mostly combined. Add in half of the sour cream/milk mixture and lightly stir until almost mixed in. Continue with adding another one-third of the flour mixture, followed by the remaining sour cream and ending with the last third of flour mixture. Mix until just combined and a smooth batter forms. Do not overmix. With a rubber spatula, give the batter one good, final stir to make sure that everything is well incorporated.
8. Remove 2 cups of the batter from the bowl & mix with the chocolate paste (recipe below).
9. Transfer the vanilla cake batter into the prepared pan, then using the back of a spoon, make a tunnel in the middle, around the pan. Spoon the chocolate batter into the tunnel, avoiding the sides. Using a butter knife, swirl the 2 batters together in an 'S' shape motion. 
10. Bake in the oven for 40 to 50 minutes, or until the center springs back to the touch and a skewer inserted down the center of the cake comes out clean or with a few moist but cooked crumbs attached.
11. Allow to cool in the pan for 20 minutes then invert on to a wire rack to cool completely. Serve plain or dust with powdered sugar or drizzle with chocolate ganache glaze if you prefer. The cake will keep for 3 days stored in air-tight container, at room temperature.

CHOCOLATE PASTE FOR THE CHOCOLATE BATTER  

Ingredients:

1/2 cup (40g) unsweetened cocoa powder - I used Valrhona.
3 tablespoons milk
1½ tablespoons butter
1½ tablespoons sugar
3/8 teaspoon baking soda

Method:

1. In a medium bowl, combine the milk, butter and sugar and heat in the microwave until the butter has melted. Whisk to dissolve the sugar. Add in the cocoa powder and stir really well until there are no lumps and a very thick chocolate paste forms. Add in the baking soda and stir again.
2. Take a few tablespoons from the reserved vanilla cake batter and stir really well into the chocolate paste to lighten it up. Add the remaining reserved cake batter into the chocolate mixture and fold until evenly incorporated and no light streaks remain.
3. For less chocolate batter (as used by Tasbih), mix 1½ cups of the vanilla batter in the following amount of chocolate paste:
   ¼ cup plus 2 tablespoons (30g/ 1⅛oz) unsweetened cocoa powder
   2 tablespoons milk
   1 tablespoon (1/2 oz, 15g) unsalted butter
   1 tablespoon sugar
   ⅛ teaspoon baking soda

Recipe by Tasbih of Cleobuttera.

Notes:

1. Using cake flour in cake baking produces a finer and more tender crumb. However, if you don't have cake flour on hand, Tasbih suggests that you replace it with a combination of 2 cups plus 3 tablespoon (7⅜ oz, 210g) of all-purpose flour & ¼ cup plus 1 tablespoon (1⅜ oz, 40g) cornstarch.
2. Tasbih used 1½ cups of the vanilla batter for the chocolate marbling while I used 2 cups. Hence, the chocolate marbling for my cake is more pronounced than Tasbih's.
3. The method of marbling used in this recipe creates a cake with subtle marbleness, meaning that the vanilla and chocolate portions are more distinct in 2 different ends, instead of them being thoroughly mixed in a zebra-like pattern. To create a more layered marbling effect, you may alternate between batter layers. Start by filling the bottom of the pan with ⅓ of the vanilla cake batter, followed by ⅓ of the chocolate batter, then cover with another third of the vanilla batter, then another third chocolate, the remaining vanilla then finishing off with the chocolate. Using a butter knife, swirl together in an 'S' shaped motion, but don't over do it.
4. Based on the volume of the batter, I suggest that you bake in a 10x10 8x8 or 9x9-inch baking pan if you are not using a bundt pan.