Preheat the oven to 375°F. Lightly spray an 8-inch square baking pan with nonstick cooking spray. Line with parchment paper, leaving an overhang on both sides for easy removal.

In a medium bowl, whisk together flour, cocoa powder, and salt until combines. Set aside.

Using a double boiler or a medium metal bowl set over a saucepan with simmering water, combine butter and dark chocolate in the top bowl. Melt the chocolate and butter together, stirring occasionally, until smooth, about 5 minutes.

Pour in sugars and whisk until combined. Add eggs in one at a time, whisking between each until combined. Remove from heat.

Whisk in vanilla extract. Pour in dry ingredients, whisking until combined.

Add in milk chocolate chips, stirring until combined.

Add batter to the prepared pan and smooth the top with a spatula.

Bake until set and a toothpick inserted in the center comes out with a few moist crumbs, about 27 to 30 minutes.

Cool brownies in the pan for at least 30 minutes before removing and slicing into 9 pieces.

Store in an airtight container for up to 5 days at room temperature.