In a medium saucepan, melt the unsalted butter over medium heat until golden brown and nutty, about 5–7 minutes. Pour into a mixing bowl to cool slightly.

Add the semi-sweet chocolate chips to the warm browned butter and stir until melted and smooth, about 2–3 minutes.

In a separate bowl, whisk together the all-purpose flour, dark cocoa powder, espresso powder, and salt.

In another mixing bowl, beat the eggs with granulated sugar, brown sugar, and vanilla extract until light and fluffy, about 1–2 minutes.

Gradually drizzle the chocolate mixture into the wet ingredients while mixing. Fold in dry ingredients until just combined.

Preheat oven to 350°F (180°C) and line an 8x8-inch baking pan with parchment paper. Pour the batter into the pan and spread evenly. Bake for about 30–40 minutes.

After baking, cool in the pan for at least 15 minutes before slicing with a hot knife.