Preheat the oven to 350°F. Spray a Bundt pan with baking spray.
In a small saucepan, simmer ½ cup water. Whisk in cocoa powder and remove from heat. Add chocolate and stir until melted.
In a medium bowl, whisk together flour and baking powder.
In a stand mixer, beat butter and sugar on low until combined. Increase speed and beat until light and fluffy, about 4 to 6 minutes. Add oil and vanilla, then the eggs one at a time.
Gradually add the flour mixture and buttermilk in alternating parts until just combined. Stir the batter again with a spatula.
Transfer half of the batter to a bowl. Add chocolate mixture to the remaining batter and stir.
Dollop alternating scoops of batters into the prepared pan. Swirl gently with a knife and tap the pan to settle.
Bake for 55 minutes to 1 hour 5 minutes, until a toothpick comes out with a few moist crumbs.
Cool in the pan for 10 minutes, then invert onto a wire rack to cool completely.
For the glaze: Whisk together powdered sugar, melted butter, buttermilk, and vanilla. Adjust consistency with more sugar or buttermilk as needed.
Drizzle the glaze over the cake and let it set for about 20 minutes before serving.
Notes
This cake is perfect for any celebration or as a delightful treat with coffee. Enjoy!