Preheat oven to 350F, grease and flour a 10-12 cup Bundt pan.
In a medium bowl, dissolve cocoa powder in 3 Tbsp hot water. Set aside.
In a medium bowl, whisk cake flour, baking powder, baking soda, and salt. Set aside .
Beat butter until smooth. Add sugar and beat on med-high until pale and fluffy (2-3mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla and mix until well combined.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of buttermilk). Fully incorporating after each addition. Do not overmix.
Add 1/4 cup of cake batter to the cocoa mixture to lighten then add an additional 1 3/4 cups to the chocolate batter.
Alternate adding chocolate and vanilla batters into the pan, swirling with a cake skewer. Smooth the top with a spatula and whack the pan against the counter to evenly distribute.
Bake for approximately 45mins or until a toothpick inserted into the center comes out mostly clean.
Place cake on wire rack to cool for 10mins. Whack the pan on the counter to loosen the cake and turn out onto wire rack to cool completely.