1In a large bowl (I used my stand mixer), beat the butter and sugars for a few minutes, until pale and light. Add the egg and vanilla and beat for another minute or two, scraping down the bowl as needed. (The mixture may look curdled - that’s OK.)
2In a small bowl, stir together the flour, baking powder and salt. Add it to the butter mixture in three additions, mixing just until blended after each.
3Divide the dough in half. Place a piece of parchment on your countertop, put one piece of the dough on it and cover with another piece of parchment. Roll until it’s about 10 inches in diameter. Turn the disc as you roll, and turn it over occasionally to ensure you aren’t rolling creases into the dough. Repeat with the other half and refrigerate the discs for a couple hours or up to a few days.
4When you’re ready to bake, preheat the oven to 350F. Place one layer, still on its parchment, onto a baking sheet. Spread with jam, leaving about half an inch around the edge. Brush with a little egg wash and top with the second piece of dough, pressing lightly around the edges to seal.
5Brush the top completely with the egg wash and run the tines of a fork across it in a crisscross pattern. Bake for 20-30 minutes, until golden, then cool on the pan on a wire rack. Cut into wedges to serve. Serves 12-16.