Heat oven to 375. Line regular muffin tins with 24 paper liners or line extra large muffin tins with 12 paper liners. To make topping, in mixer, fitted with paddle attachment, mix the brown sugar, flour and cinnamon and mix on low speed. Add butter and mix on low speed til crumbly. Mixture will look sandy at first, and then will bind into crumbs. This may take several minutes, so keep mixing til larger crumbs form. Add the chocolate chips and mix just til they are dispersed. You can make this topping in advance and keep it in an airtight container in refrigerator for up to a week. To make muffins, sift together flour, cocoa powder, baking powder and baking soda and salt in a bowl. Set aside. In bowl of mixer, (no need to wash the bowl after making the topping) fitted with whip attachment, beat butter on medium speed til it is light and fluffy. Then add sugar and mix well. Mix in the eggs, one at a time, and then mix in the vanilla. Add a third of the dry ingredients and mix til barely combined. The mix in a third of the sour cream. Repeat til all dry ingredients and sour cream are added and mixed til just combined. Scoop batter into lined muffin tins, filling them about halfway. Spoon topping onto tops of the muffins. Bake til muffins are cooked through and the topping is golden brown, 20 to 25 minutes for regular muffins, 25 to 30 minutes for extra large muffins. Serve warm or at room temperature.
Makes 24 regular or 12 large muffins