In a medium bowl, combine the flour, baking powder, and salt; stir briefly to mix and set aside.
To the bowl of a stand mixer fitted with a paddle attachment, add the butter and sugar; beat on medium high speed until light and airy, about 3 minutes. Scrape down the bowl and add 1 egg plus the egg yolk; beat on medium-low speed to combine. Add the flour mixture in two batches, beating on low speed until just incorporated each time.
Remove the bowl and form the dough into 2 equal balls using your hands (do not overhandle). Place 1 ball between two large sheets of parchment paper, and roll out the round to about 11 inches in diameter; place on a baking sheet (do not remove the paper). Repeat the process with the other ball of dough, rolling it out to about 9 inches in diameter. Stack the dough disks on the baking sheet and refrigerate until firm but still pliable, 30 minutes to an hour.
Set a rack in the top third of the oven and preheat to 375°F. Grease a 9-inch round cake pan with butter. Add a parchment round to cover just the bottom of the cake pan.
Remove the dough rounds from the refrigerator; let rest at room temperature for about 5 minutes. Carefully peel away the paper on both sides of the larger round. Place the dough in the prepared pan and press it gently into the edges of the bottom; trim so the dough reaches halfway up the sides. Spoon the cherry preserves in the center of the dough, and spread with the back of the spoon to coat evenly, leaving a 3⁄4-inch border around the edges. Remove the paper on both sides of the smaller dough round, then center it atop the cake pan. Trim the edges so the dough just fits inside, but still touches the sides of the pan. Using fingers, press down around the edges, tucking the dough edges in and fusing the two rounds together.
In a small bowl, beat the remaining egg with a few drops of water. Brush the top of the dough generously with the egg wash. Then, using a small, sharp knife, create a pattern of crisscrossed lines.
Bake until the top is golden, 30–35 minutes. Remove to a rack and let cool 5 minutes. Use a paring knife to dislodge the cake’s edges from the pan. Invert the cake onto a flat plate, then invert again onto another plate or stand. Let cool completely before slicing into wedges to serve.