In oven: Wrap whole beets (stems removed) in aluminum foil and roast in 400° F oven for at least an hour until soft to the touch. OR, In Instant Pot: Set beets on rack and add 1 cup water. Cook on high pressure for 15 minutes. Instant release pressure. Allow beets to cool enough to handle and peel away skin.
Puree beets in blender or food processor with just enough buttermilk to allow puree to achieve a very smooth consistency. If you end up with extra puree, you can use extra puree in pancake batter, soups, etc.
Preheat oven to 325F. Grease or line a 8x8x2 metal pan with parchment foil paper.
Using a medium to large saucepan over medium heat, melt butter. Stir frequently as it continues to bubble. When there are small brown specs, and it smells like nutty caramel, remove pan from heat.
Add sugar, cocoa powder and salt to combine.
In separate bowl, beat eggs. After butter mixture has cooled in pot 4-5 min, slowly pour eggs into pot with sugar/butter/cocoa powder, stirring constantly until well combined.
Add vanilla and beet puree, stir to combine.
Add flour and baking powder, stir to combine.
Add chocolate chunks, stir to combine.
Bake 25-35 minutes in preheated oven or until top no longer looks wet. Enjoy!