Preheat your oven to 350°F (175°C). Grease and flour a 10-inch bundt pan very thoroughly.
In a large bowl, whisk together the flour, baking powder, baking soda, and salt.
In a separate large bowl, cream together the softened butter and sugar until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the sour cream and milk. Begin and end with the dry ingredients. Mix until just combined.
Divide the batter in half. In one half, stir in the cocoa powder until well combined.
Spoon alternating dollops of vanilla and chocolate batter into the prepared bundt pan.
Using a knife or skewer, gently swirl the batters together to create a marbled effect.
Bake for 50-60 minutes, or until a wooden skewer inserted into the center comes out clean.
Let the cake cool in the pan for 10-15 minutes before inverting it onto a wire rack to cool completely.
While the cake is cooling, prepare the chocolate glaze. In a heatproof bowl set over a saucepan of simmering water (or in the microwave in 30-second intervals), melt the chocolate and butter together, stirring until smooth.
Remove from the heat and stir in the milk and vanilla extract. Gradually add the sifted powdered sugar, mixing until smooth and glossy.
Once the cake is completely cool, pour the chocolate glaze over the top, allowing it to drip down the sides. Let the glaze set before serving.