Cocoa and walnuts cupcakes

Ingredients:

2 1/2 cups self rising flour
1/4 cup unsweetened cocoa powder
1  teaspoon baking powder
1 teaspoon   bicarbonate of soda
1 teaspoon salt
1 cup buttermilk 
1 teaspoon white vinegar
1 teaspoon vanilla extract
1/4 pound unsalted butter, at room temperature
1 1/2 cups sugar
2 large eggs 
1/2 cup walnuts chopped

Directions:

Preheat the oven to 175 ° C. Line muffin tins 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.
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 25 minute until a toothpick inserted in the centers comes out clean.