Preheat oven to 350ºF. Line a 9-inch round cake pan with a parchment paper round that has been greased and floured; set aside.
In the bowl of a food processor, add cooked beets and process until puréed cleaning the sides and bottom of the bowl as needed. Transfer to a measuring cup (amount needed is roughly 1 cup, more or less) and set aside.
In a large bowl, add flour, cocoa powder, baking powder, baking soda, and salt; whisk until well combined and set aside.
In the bowl of a stand mixer, add butter and sugar. Using the paddle attachment, cream the ingredients on medium speed until fluffy and pale in color, about 5 minutes.
Add eggs, one at a time, and beat well between each addition.
Add beets and vanilla extract. Process until blended, cleaning the sides and the bottom as needed.
Add half the dry ingredients and half the buttermilk. Process on low speed until just incorporated before adding the remaining dry ingredients, buttermilk and oil; process until just combined - don’t overmix.
Pour batter into the prepared cake pan and gently tap on the counter to dislodge any trapped air bubbles.
Transfer to the preheated oven and bake for 55 to 65 minutes or until a cake tester inserted in the center comes out clean with just a few crumbs.
Remove from the heat and allow the cake to cool in the pan on a wire rack for 20 minutes. Invert the cake, discard parchment paper and cool completely on a wire rack.