Preheat your oven to 350°F (175°C). Line an 8x8 inch (20x20 cm) baking pan with parchment paper, leaving an overhang on two sides to easily lift the brownies out later. Lightly grease the parchment paper.

In a large mixing bowl, combine the melted unsalted butter, granulated sugar, and light brown sugar. Whisk vigorously for about 1-2 minutes until smooth and slightly thickened.

Add the eggs one at a time, whisking well after each addition until fully incorporated. Stir in the vanilla extract.

In a separate medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, and salt. Gradually add the dry ingredients to the wet ingredients, folding gently with a rubber spatula until just combined. Be careful not to overmix.

Fold in the semi-sweet chocolate chips (if using). Pour the batter into the prepared baking pan and spread evenly.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs, but not wet batter. Do not overbake. Let the brownies cool completely in the pan on a wire rack before frosting.

While the brownies cool, melt the 1/4 cup unsalted butter in a small saucepan over low heat. Remove from heat and whisk in the 3 tbsp unsweetened cocoa powder until smooth.

Gradually whisk in the sifted powdered sugar, alternating with the milk, until smooth and well combined. If the frosting is too thick, add a tiny bit more milk, 1 teaspoon at a time, until it reaches a spreadable consistency. Stir in the vanilla extract and a pinch of salt.

Once the brownies are completely cool, pour the prepared fudge frosting over the top. Use an offset spatula or the back of a spoon to spread the frosting evenly to the edges.

Immediately sprinkle the candy-coated chocolate sprinkles generously over the wet frosting. This ensures they stick properly.

Allow the frosting to set for at least 20-30 minutes at room temperature, or briefly in the refrigerator, before lifting the brownies out of the pan using the parchment paper overhang. Cut into 16 squares.