Blueberry Sour Cream Cake - cooking recipe

Ingredients

1/2 cup sweet butter (unsalted)
1 cup sugar
3 eggs
2 cups sifted flour
1/2 teaspoon salt
1 teaspoon baking powder
1 teaspoon baking soda
1 cup sour cream
1 teaspoon vanilla extract
1/2 cup packed brown sugar
2 cups blueberries (fresh is best)

Preparation

Preheat oven to 325 degrees.
Generously grease a 9x13x2 pan.
Cream butter and sugar.
Add eggs, one at a time, beating well after each addition.
Sift dry ingredients together and gradually add egg mixture, alternating with sour cream and ending with flour mixture (this is the part I'm not understanding).
Stir in 1 cup blueberries, and add vanilla. Pour 1/2 of the batter in the pan and top with remaining cup of blueberries, sprinkle with brown sugar.
Top off with remaining batter and bake for 45-50 minutes.
Cool in pan (on a rack) for 25-30 minutes.