Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan and line it with parchment paper for easy removal.
In a medium saucepan, melt the butter over low heat. Once melted, remove it from the heat and let it cool slightly.
In a large mixing bowl, combine the granulated sugar, brown sugar, and melted butter. Stir until well combined.
Add the eggs one at a time, mixing well after each addition. Then, stir in the vanilla extract.
In another bowl, whisk together the flour, cocoa powder, baking powder, salt, and instant espresso powder.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Don't overmix.
Fold in the chocolate chips and chopped nuts (if using) until evenly distributed throughout the batter.
Pour the batter into the prepared baking pan, spreading it evenly with a spatula.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).
Allow the brownies to cool in the pan for about 15 minutes before lifting them out using the parchment paper. Let them cool completely on a wire rack before slicing.
Notes
Serve warm with vanilla ice cream for extra indulgence.