1. Thicken jam (see notes above) and leave to cool.
2. Whisk flour, baking powder and salt in a medium bowl, set aside.
3. In a large bowl, using an electric mixer on medium, beat butter and sugar until light and fluffy. Add 2 eggs only, one at a time. Scrape down the sides of the bowl after each egg is incorporated and beat again.
4. On low speed, slowly add in the dry ingredients. Mix just until dough sticks together and everything is well incorporated. Divide into four balls and press flat so they each resemble a disc. Refrigerate for 30 minutes. While the dough is chilling, line 2 baking trays with parchment paper, flour your surface, whip the last egg for the egg wash and turn the oven on to 350°F.
5. Roll out each disc on a lightly floured surface to about 1/4 in. thickness. The dough will crack in some areas, and it will also stick lightly to your surface. Using a 3 1/2 in. cookie cutter, cut as many rounds as you can. Use a pastry scraper or fish spatula to remove rounds from the surface and place them on parchment paper.
6. Fill each round with a teaspoon of jam and follow these folding instructions. Only work on one disc at a time. If you cut all of them and fill them at once, the dough will start to dry out before you fold, making this step more difficult.
7. You’ll end up with scraps of dough. Roll them back into a ball and roll it out again to make more rounds. If the dough cracks when you’re folding, use your fingers to seal any cracks the filling can leak out of.
8. Once all the cookies are filled and folded, brush the top lightly with the egg wash making sure to seal any corners and cracks.
9. Bake the cookies for 10 minutes, rotate the baking sheets, then cook for 10 more minutes or until bottoms are golden brown. Cool on baking sheets. Filling may seep over some cookies—about three of my 24 did with this method, which was a big improvement from years past. And be careful—the filling gets extremely, dangerously hot, so don’t eat them straight out of the oven.