Preheat the oven to 350 degrees, and line a 12 cup muffin pan with cupcake liners.
In a large mixing bowl, with the paddle attachment, cream together the butter and sugar. Add the eggs, one at a time, beating between each addition. Then, add the pumpkin.
Sift together the flour, baking powder, baking soda, salt, and pumpkin spice, and stir them very gently into the batter. Do not overmix. Add the sour cream and vanilla, until just combined.
Divide the batter between the 12 muffin cups.
To make the streusel, mix together the sugar, flour, and pumpkin spice. Then, with a pastry cutter or your fingers, cut in the butter, until large crumbs are formed. Divide the streusel between the tops of each muffin.
Bake twenty minutes or until a toothpick comes out clean. Cool in the pan for five minutes, and then on a wire rack for ten minutes.