In a large bowl, sift together the flour, sugar, cocoa powder, baking soda, baking powder, and 1 teaspoon kosher salt. In a separate bowl, whisk together the buttermilk, eggs, canola oil, vanilla, and beet puree. Stir the wet ingredients into the dry ingredients and mix until well combined. Transfer the batter to the prepared cake pan and bake until a tester inserted into the center of the cake comes out clean, 55 to 60 minutes.