Preheat oven to 325F and grease a 10-cup bundt pan with cooking spray.
In a medium bowl, whisk flour and baking powder, set aside.
In a stand mixer fitted with the paddle attachment, cream butter and sugar on med-high until pale and fluffy (approx 5mins).
Add eggs one at a time, fully incorporating after each addition. Add vanilla.
Reduce speed to low. Add flour mixture in 3 portions, alternating with 2 portions of milk. Mix until just combined.
Separate the batter into 3 bowls.*
Add 2 Tbsp Dutch-processed cocoa powder to one and 2 Tbsp black cocoa to another. Mix each until well combined.
Prepare 3 separate pastry bags and fill each with one of the batters.
Pipe the batter, alternating the colors, into the cracks of the Heritage bundt pan, being careful to avoid spilling over into neighbouring cracks. **
Once cracks are sufficiently filled, add remaining batter in alternating layers and use a kitchen skewer to marble it. Spread the batter evenly.
Bake for 55-60mins or until a skewer comes out clean.
Remove from oven and transfer bundt pan to cooling rack for 10-15mins. Rap the bundt pan against the counter to loosen it then turn cake out onto cooling rack to cool completely.
Combine powdered sugar and milk in a medium measuring cup. Stir until well combined. Add more milk as needed until desired consistency is reached. Pour glaze into cracks of cooled cake.
*I use a kitchen scale to weigh the batter to ensure I had even amounts, but a normal person would probably just eyeball it. **I used a kitchen skewer for the finer points of the cracks. It's a process. Note: Prep time is 45mins assuming you're doing the colored spirals. Otherwise it would take 20mins or so.NOTE: Measurements in grams have not been tested. An online tool was used to convert these measurements.Adapted from Add a Pinch.