Mix the dry ingredients together, cut in the cold butter with a pastry blender, your fingers or a food processor until you have small pea sized lumps of butter.
Set the streusel aside until the muffin batter has come together.
Preheat the oven to 350F.
Cream the butter and sugar together, add the vanilla, eggs and sour cream, mix well.
Mix in the dry ingredients until just combined.
Line muffin cups with papers and fill each cup a little less than half way, sprinkle with 1 tablespoon of streusel, add another scoop of batter and top with the remaining streusel, about 1 heaping teaspoon.
Bake at 350F for 25-30 minutes or until a toothpick inserted in the center comes out clean.