Let the butter melt in a light-colored saucepan over medium heat. Once melted, keep cooking for around 4-5 minutes, stirring regularly, until the milk solids have caramelized and the butter smells nutty. Pour the browned butter into a large heatproof bowl (including all the dark bits from the bottom of the pan) and let it cool for 15 minutes. 170 g butter
Preheat the oven to 350°F (180°C). Line an 8x8-inch (20x20cm) baking pan with parchment paper.
Once the butter has cooled, add the brown sugar, egg, espresso powder, and vanilla. Whisk until smooth and well combined. 200 g soft brown sugar, 1 large egg, 1 tablespoon instant espresso powder / coffee powder, 1 teaspoon vanilla extract
In a small bowl, whisk together the flour, baking powder, and salt. 190 g all-purpose flour, 3/4 teaspoon baking powder, ½ teaspoon salt
Add the dry ingredients to the wet ingredients and fold with a rubber spatula until combined into a thick, glossy batter. Fold through the chocolate chips. 170 g dark chocolate chips
Press the blondie dough into the prepared baking pan and smooth with an offset spatula. Press a few extra chocolate chips or chunks on top.
Bake for 25-28 minutes, or until the middle has set and a toothpick inserted into the center comes out with a few moist crumbs. Don't overbake for fudgy blondies.
Let cool completely in the pan on a wire rack. Sprinkle with flaky salt to serve (optional).