For the streusel topping:
1/4 c. light or dark brown sugar
1/4 c. all-purpose flour
1/4 tsp. nutmeg
1/4 c. unsalted butter, softened
For the muffin batter:
1/2 c. unsalted butter, softened, plus additional for greasing pan
1 1/4 c. white sugar
2 eggs, beaten
1/4 c. sour cream
3/4 c. milk
1 tsp. lemon zest
1 1/2 c. all-purpose flour
2 tsp. baking powder
1/2 tsp. salt
2 c. blueberries, fresh or frozen
Preheat oven to 375°F/190°C. Butter a 12-c. muffin pan or line with paper liners and set aside.
Prepare the streusel topping by combining brown sugar, flour and nutmeg in a small bowl. Cut in the butter with a fork until a mealy texture develops. Set aside.
To prepare the muffin batter, cream butter and white sugar in a large bowl until fluffy. Beat eggs into mix, one at a time. Add sour cream and milk; stir in the lemon zest.
In a separate bowl, sift together the flour, baking powder and salt.
Stir the dry ingredients into the wet ingredients until just combined. Do not over mix. The batter will have some lumps. Gently fold the blueberries into the batter.
Pour the muffin batter into the muffin pan, filling only 2/3 of each muffin cup. Sprinkle each muffin with some of the streusel.
Bake until golden brown on top, about 25 - 30 minutes. Cool slightly and remove muffins from pan while still warm.