Step 1

In a large mixing bowl, or stand mixer fitted with paddle attachment, cream together granulated sugar and butter until light and fluffy

Step 2

Add eggs, one at a time, mixing for 30 seconds after each addition.

Step 3

In a medium-sized mixing bowl, sift together all-purpose flour and baking powder.

Step 4

Add the flour mixture, vanilla extract and sour cream to the butter mixture. Gently fold in until combined.

Step 5

Spoon half of the batter in a greased Bundt cake pan.

Step 6

To the remaining batter, add in the unsweetened cocoa powder and stir just until combined.

Step 7

Spoon the chocolate mixture on top of the vanilla batter in the Bundt pan.

Step 8

Using a knife, swirl the two batters together to create a marbled effect.

Step 9

Bake in 325°F oven for 50 to 60 minutes until tester inserted in centre comes out clean.

Step 10

Cool on wire rack for 10 minutes, then remove from pan and cool completely on wire rack.

Step 11

In a small saucepan, over low-medium heat, melt butter and chocolate chips stirring occasionally until melted being careful not to overheat the mixture.

Step 12

Remove from heat and stir in powdered sugar, beating until well combined. Add additional powdered sugar until spreadable or pourable consistency is achieved.

Step 13

Spread or pour over top of cake. To frost the entire cake, make a double batch of the glaze.