Thoroughly butter the bottom and sides of a 9x9-inch pan.
In a medium bowl, combine flour, sugar, baking powder, and salt. Add butter and mix until crumbly and resembling coarse cornmeal.
Press 2/3 of the flour mixture into the bottom of the prepared pan, smoothing a flattening to create an even base. Chill for at least 30 minutes. Meanwhile, preheat oven to 375°F.
Remove base from refrigerator and spread jam evenly over the top. Crumble the remaining flour mixture over the jam layer. Bake until the jam is beginning to bubble and the crumble is beginning to brown, about 35 minutes. Let cool completely before slicing.