1cupsour cream or buttermilkI used 1/2 cup sour cream, 1/2 cup buttermilk
for the streusel
Preheat oven to 350F
Spray a 12 cup muffin pan with cooking spray, or use muffin liners.
Mix the streusel in a small bowl and set aside. Make sure your walnuts are finely chopped, and that you get the lumps of brown sugar broken down.
Mix the flour, baking powder, baking soda, salt and cardamom in another small bowl and set aside.
Measure out the sour cream or buttermilk, or mix the two if you are using both, and set aside.
Cream the butter and brown sugar until fluffy.
Beat in the eggs, one at a time. Blend in the vanilla.
Fold the flour into the butter mixture in thirds, alternating with the sour cream/buttermilk. Just stir enough to blend, don't beat or over mix.
Spoon a small scoop of batter into the bottom of 12 muffin cups. Top with a teaspoon or so of streusel mix. Scoop the remaining batter on top and then top with another teaspoon of streusel, use up all the streusel.
Bake for about 20 minutes until a toothpick comes out clean.
Cool for a few minutes before removing them from the pan to a rack.