Prepare the Pan: Preheat the oven to 350°F (175°C). Grease and flour a 10-inch Bundt pan thoroughly, ensuring all crevices are well coated.
Mix Dry Ingredients: In a small bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
2 ½ cups (300g) all purpose flour, 1 ½ teaspoon baking powder, ½ teaspoon baking soda, 1 teaspoon salt
Cream Butter and Sugar: In a large mixing bowl, beat the butter and sugar together on medium speed until light and fluffy (about 3-4 minutes).
1 cup (226g) unsalted butter, 2 cup (200g) white granulated sugar
Add Eggs and Vanilla: Beat in the eggs, one at a time, ensuring each is fully incorporated before adding the next. Mix in the vanilla extract.
4 large eggs, 1 tablespoon vanilla extract
Alternate Wet and Dry: On low speed, add the dry ingredients in three batches, alternating with sour cream and milk, beginning and ending with the flour mixture. Mix only until just combined after each addition. (1/3 flour, milk, 1/3 flour, sour cream, 1/3 flour)
1 cup (275g) sour cream, ½ cup whole milk
Make the Chocolate Batter: In a medium bowl, transfer 1/3 of the vanilla batter into it and mix in the cocoa powder and hot water
¼ cup (30g) cocoa powder, 3 tablespoon water
Layer and Swirl: Spoon half of the remaining vanilla batter into the prepared bundt pan. (Optional) Smooth the batter and create little waves so that the layers aren't so straight. Add spoonfuls of chocolate batter on top, then cover with the remaining vanilla batter. Using a skewer or knife, gently swirl the batters together to create a marbled effect. Bake the Cake: Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for 10-15 minutes, then turn it out onto a wire rack to cool completely.