In a bowl, whisk together the flour, baking powder, and salt. Set aside. Preheat the oven to 350°F (175°C).
Line 13 by 18-inch (33 by 46 cm) baking sheet with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar, and powdered sugar on medium until soft and creamy, pausing halfway through to scrape the sides and bottom of the bowl with a rubber spatula, 2 to 3 minutes.
Add the flour mixture to the butter mixture and mix until all the ingredients are well incorporated. The dough will be fairly thick. Divide the dough into two equal portions.
Lightly dust a work surface with flour, then roll each ball of dough into a log about 12-16 inches (30 cm) long. Cut each log into 8-12 cookies at 1½-inch (3.5 cm) intervals and arrange them on the prepared baking trays, spaced 2 to 3 inches (5 to 7.5 cm) apart.
Use your thumb to make a careful depression in the top of each cookie. Your thumb should come about ¼ inch shy of the baking tray. Be careful not to push all the way through the dough. As you press down, the cookie will expand into a rounded shape. Don’t worry if the edges of the dough crack a little.
In a small bowl, whisk the egg with a fork. Using a pastry brush, lightly brush each cookie with the egg wash, focusing on the edges. Sprinkle the tops with the demerara sugar.
Drop a heaping teaspoon of the jam into the center of each cookie.
Bake for 17 to 20 minutes, rotating the baking trays halfway through from top to bottom and front to back, until golden at the edges and the jam is set. Transfer them to a rack to cool completely. Store the cookies in an airtight container, where they will stay fresh for up to 3 days.