Preheat oven: Preheat your oven to 350°F (180°C). Grease and flour a 10-inch (25 cm) bundt or Gugelhupf pan.
Mix dry ingredients: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
2 cups All-Purpose Flour, 2 tsp. Baking Powder, ½ tsp. Salt
Cream butter and sugar: In a large bowl, cream the butter and sugar until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
1 cup Unsalted Butter, 1+¼ cup Granulated Sugar, 4 large Eggs, 1 tsp. Pure Vanilla Extract
Alternate dry ingredients and milk: Gradually add the dry ingredients to the butter mixture, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.
¾ cup Whole Milk
Divide and flavor the batter: Divide the batter evenly between two bowls. Stir the cocoa powder into one of the bowls, mixing until fully incorporated.
¼ cup Unsweetened Cocoa Powder
Create the marbled effect: Spoon dollops of the vanilla and chocolate batters alternately into the prepared pan. Use a butter knife or skewer to swirl the batters together gently, creating a marbled effect.
Bake: Bake for 45-55 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Cool: Allow the cake to cool in the pan for about 10 minutes before turning it out onto a wire rack to cool completely.
Prepare the chocolate glaze: Place the chopped chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer. Pour the hot cream over the chocolate and let it sit for 1-2 minutes. Add the softened butter and whisk until smooth and glossy.
4 oz. Semi-Sweet Chocolate, ½ cup Heavy Cream, 2 tbsp. Unsalted Butter
Glaze the cake: Once the cake has cooled completely, place it on a serving plate or cake stand. Slowly pour the chocolate glaze over the cake, allowing it to drizzle down the sides. Let the glaze set for a few minutes before slicing and serving.