Chocolate-flecked Layer Cake with Milk Chocolate Frosting

• 2 1/4 cups plain flour
• 1 1/2 teaspoons baking soda
• 1 teaspoon salt
• 1 1/2 sticks unsalted butter, softened
• 1 cup white sugar
• 1/3 cup firmly packed brown sugar
• 1 large egg yolk
• 2 large eggs
• 1 tablespoon pure vanilla extract
• 1 1/2 cups sour cream
• 3/4 cup milk
• 200g (about 1 cup) dark chocolate, finely chopped

Milk chocolate frosting

• 300g (about 1 1/2 cup) milk chocolate, finely chopped
• 2 1/2 sticks unsalted butter, softened
• 1 cup pure icing sugar, sifted
• Pinch of salt
• 2/3 cup sour cream

Preheat oven to 350°F. Grease two 8" cake pans (2.5 to 3" deep) and line with baking paper.

Whisk flour, baking soda and salt in a medium bowl. Using an electric mixer, beat butter and sugars for 5 minutes or until light and fluffy. Add egg yolk and eggs, 1 at a time, beating well after each addition. Beat in vanilla. Reduce speed to low. Gradually add flour mixture in 3 batches, alternating with sour cream and milk, beginning and ending with flour. Fold in chocolate.

Divide batter between prepared pans. Level evenly with a spatula. Bake for 40 to 45 minutes, swapping position of cakes halfway through cooking, until deep golden brown in color and a skewer inserted in center of cakes comes out clean. Cool cakes in pans for 20 minutes. Turn cakes onto a wire rack, topside up. Cool completely.

Meanwhile, make Milk chocolate frosting: Melt chocolate in a heatproof bowl set over a saucepan of simmering water. Cool for 5 minutes. Using an electric mixer, beat butter, sugar and salt for 5 minutes or until light and fluffy. Gradually beat in chocolate, then sour cream, beating until well combined (frosting should be spreadable, see notes).

Cut each cake in half horizontally with a serrated knife. Place 1 bottom layer on a platter or cake stand. Spread evenly with 3/4 cup frosting. Repeat with a second layer and another 3/4 cup frosting. Repeat layering. Spread top and side of cake with remaining frosting, smoothing top and sides. Serve immediately.