In a medium-sized microwave-safe bowl, or in a saucepan set over low heat, melt the butter, then add the sugar and stir to combine. Return the mixture to the heat (or microwave) briefly, just until it's hot (about 110°F to 120°F), but not bubbling; it'll become shiny looking as you stir it. Heating this mixture a second time will dissolve more of the sugar, which will yield a shiny top crust on your brownies.
Stir in the cocoa, salt, baking powder, and vanilla. Whisk in the eggs, stirring until smooth; then add the flour, chips, espresso powder, and nuts, again stirring until smooth. Spoon the batter into a lightly greased 9 x 13-inch pan.
Bake the brownies in a preheated 350°F oven for 28 to 30 minutes, until a cake tester inserted into the center comes out clean, or with just a few moist crumbs clinging to it. The brownies should feel set on the edges and in the center (barely). Remove them from the oven and cool on a rack before cutting and serving.