Preheat oven to 350℉. Grease mini donut pans and set aside.
Whisk together flour, cake flour, cocoa powder, baking soda, and salt. Sift cake flour and cocoa powder if clumpy Set aside.
Whisk together buttermilk, vinegar, and vanilla in another bowl. Set aside.
Cream butter and sugar in a bowl of stand mixer fitted with a paddle attachment 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 until batter is smooth.
Transfer batter to piping bag with small tip. Fill donut pans a little less than ⅔ full. Do not overfill.
Bake for 10-15 minutes until donut springs back when touched. Let cool in pan for 2 minutes before unmolding.