Use a mixer to beat the softened butter and sugar. Then add eggs one at a time and beat for another minute or a half.
Mix flour with baking soda and cocoa. Add to the butter and egg mixture and mix well.
Add yogurt or low-fat sour cream and stir to form a homogeneous mass.
We will bake in a 30 x 30 cm form. Cover the form with parchment paper and pour out the dough. We level and put the form in an oven preheated to 180 degrees for 20 minutes.