Preheat oven to 400° F.
To make the beet puree, wrap the whole beet tightly in foil and cook in oven for at least an hour until soft. When cool enough to handle, remove skins. Combine with the minimum amount of buttermilk in blender to create a smooth puree.
In small bowl, mix flour, cocoa powder, baking soda, and salt.
In the bowl of your stand mixer with the paddle attached, beat butter and sugar on medium speed until well blended. Add beet puree and vanilla; mix well and then scrape sides. Mix in egg, mixing well. Add flour mixture, and beat slowly to incorporate, then faster to blend well. Stir in chocolate chunks.
Drop batter in 2-tablespoon portions about 2 inches apart onto lined baking sheets.
Bake in preheated oven until edges of cookies are brown, about 9 minutes.
Let cookies cool on pan about 5 minutes, then transfer to cooling racks with a spatula. Serve warm or cool. Store in airtight container, or freeze for longer storage.