SIFT flour with baking powder and salt. Cream butter. Gradually add sugar, creaming well after each addition. Stir in vanilla and egg. Beat well.
ADD flour mixture alternately with milk, beating after each addition until smooth.
SPOON batter into greased and floured muffin pans, filling each about 2/3 full. Place 1/2 tablespoon of blueberry jam on the center of each muffin and press lightly into batter.
BAKE for 25 minutes at 375 degrees. Cool in pan for 10 minutes. Makes 8 to 12 cupcakes.