Heat the oven to 350?F with a rack in the center.

Cut the butter into 4 pieces and place in a large microwave-safe bowl. Microwave in 10-second intervals, stirring between each, until the butter is mostly melted with a few small pieces remaining, about a minute total. Set aside to cool for a few minutes. Line an 8x8-inch baking dish with parchment paper, leaving a 2-inch overhang on two opposite sides.

Add the brown sugar, egg, and vanilla extract to the bowl of melted butter; stir until combined and smooth. Add the flour, instant espresso, baking powder, and salt, and mix until just combined. Fold in the chopped toffee.

Transfer the batter to the prepared baking pan and press into an even layer. Bake until the edges are firm and a toothpick inserted in the center comes out mostly clean with just a few crumbs, 20 to 25 minutes.

Let cool in the pan on a wire rack for at least 20 minutes. Use the parchment sling to lift the blondie slab out of the pan and onto a cutting board. Cut into 16 squares. Leftovers can be stored in an airtight container at room temperature for up to 5 days.