Position rack in center of oven. Preheat oven to 325°F/165°C. Coat an 8-inch (20 cm) square pan with non-stick spray. Line with a strip of parchment paper to cover bottom and overhang on two sides. Spray the paper, too.

Whisk flour, baking powder and salt in a small bowl to aerate and combine. Add chopped chocolate and toss to coat; set aside.

Whisk together the melted butter, sugar, brown sugar and vanilla. Add eggs one at a time, whisking well after each addition. Stir in instant espresso. Stir dry ingredients into the butter/espresso mixture until just combined. Scrape into prepared pan.

Bake for about 45 to 55 minutes, or until bars are slightly puffed and the edges have begun to come away from the sides of the pan. A toothpick inserted in the center will show a few moist crumbs clinging. Cool on rack before cutting. Cut into 25 bars (5x5 grid). Store at room temperature in airtight container for up to 4 days.