Preheat the oven to 350°F. Lightly grease a 9" x 13" pan., Melt the butter, and stir in the brown sugar. Add the salt, vanilla, and flavor, stirring until well combined., Allow the mixture to cool slightly (if it's very hot to the touch), then add the eggs one at a time, beating well after each addition. Scrape the sides and bottom of the mixing bowl midway through this process., Add the baking powder, espresso powder, and flour, stirring to combine., Stir in the chips or other additions of your choice., Scoop the batter into the prepared pan, spreading it to the edges with a wet spatula (or your wet fingers). Smooth the top as best you can., Bake the bars for 30 to 32 minutes, until they've risen, and their top is shiny and golden. Bake for the shorter amount of time in a darker pan; the longer time in a light/shiny pan. A cake tester inserted into the center won't come out clean; in fact, the center of the very middle of the pan may look quite molten when you dig into it. But so long as no wet batter is showing farther out towards the edges, the bars are done. As they cool, the center will solidify., Remove the bars from the oven, and use a heatproof spatula to press down the risen edges; this will make bars with flat, rather than humped tops. Let the bars cool completely before cutting; overnight is best. Once bars are cool, wrap airtight. Store wrapped bars at room temperature for a couple of days.