Homemade Samoas Cookie Bars

Preheat oven to 325 ºF. Line a 13-by 9-inch metal baking pan with parchment paper, leaving an overhang on two sides. In a medium bowl, whisk together the flour, baking powder and salt. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar on medium speed until light and fluffy, about 1 minute. Turn the mixer to low, and then carefully add the flour mixture to the bowl, beating just until combined and scraping down the sides as needed. (Do not overmix the shortbread dough or it'll be too crumbly to cut once baked.) Transfer the dough into the prepared baking pan and press it into an even, flat layer. Bake the shortbread until barely golden, 15 to 18 minutes. Transfer the pan to a wire rack and let it cool completely while you make the topping.