Preheat oven to 350º and line two baking sheets with parchment. In a large bowl, whisk together flour, baking powder, and salt. In another bowl, beat butter and sugar until pale and fluffy, about 3 minutes. Beat in egg and vanilla, then add dry ingredients in two batches until incorporated. Using a small cookie scoop, scoop 1” balls onto prepared baking sheets. Press a thumbprint into center of each ball, 1/2” deep. Fill with a small spoonful of jam. Bake until edges of cookies are golden, 13 to 14 minutes. Cool on baking sheets before serving.