Preheat your oven to 350°F (175°C). Grease and flour a 9-inch bundt pan thoroughly.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In a large mixing bowl, beat the butter and sugar with an electric mixer on medium-high speed until light and fluffy, about 3-4 minutes.

Beat in the eggs one at a time, making sure each is fully incorporated before adding the next. Mix in the vanilla extract.

Gradually add the dry ingredients to the butter mixture, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.

In a small bowl, mix the cocoa powder and hot water until smooth. Add to 1/3 of the batter and mix until fully incorporated.

Spoon half of the vanilla batter into the prepared bundt pan, followed by the chocolate batter, and top with the remaining vanilla batter. Use a knife to swirl the batters together for a marbled effect.

Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean. Rotate the pan halfway through baking for even cooking.

Remove the cake from the oven and let it cool in the pan for 10 minutes. Then turn it out onto a wire rack to cool completely.

Once cooled, slice and serve the cake plain or with a dusting of powdered sugar or a drizzle of chocolate glaze.

Store leftover cake in an airtight container at room temperature for up to 3 days, or refrigerate for longer freshness.

For longer storage, wrap the cake tightly in plastic wrap and freeze for up to 3 months. Thaw overnight in the refrigerator before serving.