Preheat the oven to 175 ° C. Line a muffin tin with paper liners.In a small bowl sift together the flour, cocoa powder, baking powder, bicarbonate and salt.
In a large measuring cup combine buttermilk, vinegar and vanilla extract.In the bowl of an electric mixer, fitted with the paddle attachment, beat the butter and the sugar until light and pale.
Add the eggs, one at a time and beat until combined. With the mixer on low speed, add the dry ingredients and the wet ingredients alternately, beginning and ending with the dry ingredients. Add the walnuts and stir with a rubber spatula to be sure the batter is mixed.
Scoop the batter into the muffin tins and bake for 15 minute until a toothpick inserted in the centers comes out clean.