"Flo Braker flavors her ethereally light shortbread with finely ground espresso beans. The best part is the golden, gooey butterscotch glaze, which becomes deliciously fudgy as the bars sit in the cookie jar."
MAKE THE GLAZE Preheat the oven to 300° and position a rack in the center of the oven. Line the bottom of a 9-by-13-inch baking pan with parchment paper.
In the bowl of a standing electric mixer fitted with the paddle, beat the butter with the sugar at medium speed until fluffy and very pale, about 5 minutes. Beat in the vanilla and salt. In a small bowl, whisk the flour with the ground espresso beans. Add the dry ingredients to the mixer in 3 batches, scraping down the side of the bowl and beating just until the dough is combined.
Press the dough into the baking pan in an even layer. Spread a sheet of plastic wrap directly onto the dough and, using a flat-bottomed glass, smooth the dough into an even layer. Remove the plastic wrap and bake the shortbread for about 50 minutes, until very lightly browned on top and firm but not solid to the touch. Transfer the pan to a rack to cool slightly, about 10 minutes.
Using a ruler, cut the warm shortbread lengthwise into 8 strips, then cut crosswise into 5 rows. Let the shortbread bars cool completely.
In a small, heavy saucepan, combine the butter, brown sugar, espresso, corn syrup and salt and bring to a boil over moderate heat, swirling the pan. Boil just until slightly thickened, 1 1/2 to 2 minutes; remove from the heat. When the bubbling subsides, immediately pour the hot glaze over the shortbread. Working quickly with a small offset spatula, spread the glaze in an even layer. Using the tip of a lightly oiled paring knife, score the glaze between the cuts, without dragging. Press an espresso bean into the center of each bar. Let cool slightly, then carefully lift out the bars and transfer to a plate.