1. Stir the flour with the baking powder and salt; set aside. Beat the butter with the sugar until fluffy. Beat in the egg and vanilla. Slowly add the flour until fully incorporated. Form the dough into two discs and wrap. Chill for at least 1 hour or overnight.
2. Preheat the oven to 350°F. Position the racks to the top and bottom thirds of the oven. Roll the dough out on a lightly floured surface to be ¼-inch thick. Using a 2-inch round cookie cutter, cut out about 26 cookies from each disc; re-roll the scraps once. Arrange half the cookies on parchment-lined baking sheets.
3. Use 1-inch alphabet cookie cutters to remove letters from 13 of the remaining cookies to spell "Happy Holidays". Cut festive shapes out of the remaining cookies; arrange on parchment-lined baking sheets. Chill for 10 to 15 minutes or until firm.
4. Bake, in batches and rotating trays once, for 15-18 minutes or until golden just around the edges. Cool completely.
Spread grape jam and strawberry spread on the bottom of the whole cookies and top with cutout cookies. Arrange the "Happy Holidays" on a board or a platter and decorate with remaining festive window cookies.
Tip: Cookies can be made in advance and held in an airtight container for up to 5 days or the freezer for up to 1 month. Fill with jam and spread up to 1 day in advance.