To make streusel topping for coffee cake, combine flour, brown sugar, cinnamon and small pieces of cold butter until small or medium clumps form. If desired, add pecans or substitute the flour for pancake mix.
A recipe for streusel coffee cake is Martha Stewart's Cinnamon Streusel Coffee Cake. Prepare the streusel by mixing together 1 3/4 cups flour, 3/4 cup light brown sugar, 1 teaspoon cinnamon and 1 teaspoon salt. Use a pastry cutter or your fingers to add in 1 1/2 sticks of cold butter, and then add in 1/2 cup of pecans. For the streusel center, combine 1/4 cup brown sugar, 1/4 teaspoon cinnamon and 1 cup of pecans.
Prepare the cake by sifting together 2 cups flour, 1 1/4 teaspoons baking powder, 1/2 teaspoon baking soda and 1/2 teaspoon salt. Use a mixer to beat together a stick of room temperature butter and 1 cup sugar for two minutes. Add 2 eggs, one at a time, and then mix in 1 1/2 teaspoons vanilla. Add the dry ingredients to the wet ingredients in three additions, alternating with 1 cup sour cream.
Pour half of the batter into a buttered 9-inch tube pan, and top with the streusel center mixture. Pour the rest of the batter into the pan, and top with the streusel topping. Bake the cake for 55 minutes at 350 degrees Fahrenheit.