How to Make It
Preheat the oven to 350°. In a food processor, combine the butter with the flour, sugar, ground espresso and salt; pulse until coarse crumbs form. Press the crumbs into a 9-by-13-inch metal baking pan to form a crust; prick all over with a fork. Freeze for 5 minutes, until firm. Bake in the lower third of the oven for 30 minutes, until the crust is lightly browned and cooked through.
In a medium bowl, whisk the flour with the cocoa, salt and baking powder. In a large saucepan, melt the butter with the chocolate over low heat. Remove the saucepan from the heat and stir in the sugar and vanilla, then stir in the eggs. Add the dry ingredients and stir until the brownie batter is smooth.
Pour the brownie batter over the shortbread crust, smoothing the surface. Bake for 30 minutes, until the brownie top is dry and lightly cracked around the edges; the center will still be a bit soft. Let cool completely before cutting into bars.