2-1/2 cups all-purpose flour
1/4 teaspoon baking powder
1/4 teaspoon salt
1 cup (2 sticks) unsalted butter, at room temperature
1/2 cup sugar
1/2 cup raspberry or strawberry jam (not jelly)
1 teaspoon vanilla
1 egg
Sift the flour, baking powder, and salt together into a small bowl and set aside. Using an electric mixer on medium-high speed, cream the butter and sugar together in a medium-sized mixing bowl until light and fluffy, about 2 minutes. Add the egg and vanilla and mix well. Add the dry ingredients and mix on low speed for several seconds. Scrape the bowl, then turn the mixer to high speed and beat until the batter is light and fluffy, about 1 minute.
Refrigerate the batter in plastic wrap or a covered container for 3 hours. Fifteen minutes before baking, preheat the oven to 275 degrees. Measure out rounded teaspoonsful of dough and roll them into balls with your hands. Place the balls about 1-1/2 inches apart on ungreased cookie sheets. Then make a firm indentation in the center of each cookie with your thumb or index finger.
Bake the cookies until lightly golden, about 25 to 30 minutes. Remove the sheet from the oven and increase the heat to 325 degrees. Place 1/2 teaspoon jam in the center of each cookie and return the tray to the oven. Bake the cookies just until the jam melts and spreads, about 10 minutes. Allow to cool before removing from tray.
