Preheat the oven to 350°F (175°C) and grease and flour an 8x8 inch baking pan or line it with parchment paper, leaving an overhang.
In a large bowl, whisk the melted unsalted butter and granulated sugar until smooth.
Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract until creamy.
In a separate bowl, whisk together all-purpose flour, unsweetened cocoa powder, chocolate whey protein powder, baking powder, and salt until lump-free.
Gradually fold the dry mixture into the wet ingredients until just combined, being careful not to overmix.
Pour in the milk and stir until the batter is uniform, then gently fold in the chocolate chips (optional).
Spread the batter evenly in the prepared pan using a spatula.
Bake for 20–25 minutes, until a toothpick inserted into the center comes out with moist crumbs but no wet batter.
Allow the brownies to cool completely in the pan on a wire rack before lifting them out using the parchment overhang.
Once cooled, cut into squares and serve. Store leftovers in an airtight container at room temperature for up to 3–4 days.