Preheat your oven to 350°F (175°C). Generously spray a 10- to 12-cup Bundt pan with baking spray to prevent sticking.

In a small saucepan, bring 1/2 cup of water to a simmer over medium heat. Whisk in the cocoa powder and remove it from heat. Stir in the chopped chocolate until melted and smooth. Set this cocoa mixture aside.

In a mixing bowl, whisk together the flour and baking powder, ensuring they’re well combined.

Using a stand mixer, beat the softened butter and granulated sugar together until fluffy and light. Add in the vegetable oil and vanilla extract, then incorporate the eggs, one at a time, mixing well after each addition.

Gradually add the flour mixture while alternating with the buttermilk until just combined. Be careful not to over-mix; you want it to remain light and airy!

Divide the batter in half. Add the cocoa mixture to one half, folding it together gently.

Dollop the batters into your prepared Bundt pan, swirling them gently with a knife or spatula.

Bake for 55 minutes to 1 hour and 5 minutes, or until a toothpick comes out clean when inserted into the cake.

Allow the cake to cool in the pan for 10 minutes before inverting onto a wire rack to cool completely.

To prepare the glaze, whisk together the powdered sugar, melted butter, buttermilk, and vanilla extract. Adjust the consistency by adding more buttermilk if necessary.

Once the cake is fully cooled, drizzle the glaze over the top and allow it to set before slicing and serving.