Preheat oven to 350 degrees F. Lightly grease mini donut pans, or butter and flour cake pans, or line muffin tins.
Whisk together flour, cake flour, cocoa powder, baking soda, and salt. Sift all dry ingredients. Set aside.
In a bowl, whisk together buttermilk, vinegar, and vanilla. Set aside.
In the bowl of a stand mixer with a paddle attachment, cream butter and sugar until pale and fluffy. Add eggs one at a time, beating well after each addition.
On low speed, add dry mixture in three additions, alternating with wet ingredients. Add desired amount of green food gel paste. After all ingredients have been added, continue to mix on high speed until batter is smooth.
If making donuts, transfer batter to piping bag with small tip. Pipe batter to fill donut cavity until half full. Bake for 10-15 minutes until donut springs back when touched. If making cakes, divide batter between two pans. Bake for 30 minutes until toothpick inserted in center of cake comes out clean