Preheat your oven to 350 degrees Fahrenheit and line a sheet pan with parchment paper to ensure the cookies come off of the pan cleanly after baking and cooling.
In a large bowl with a hand mixer or in the bowl of a stand mixer that is fitted with a paddle attachment, beat room temperature butter, packed light brown sugar, and granulated sugar together until smooth and combined. This should be about 2-3 minutes of mixing.
Add the egg, milk, and vanilla extract to the bowl and beat on medium speed until smooth.
Pour cocoa powder, properly-measured all purpose flour, baking soda, and salt into the dough. Beat on low speed until smooth. Do not mix past when you see the dry ingredients fully disappear to avoid dense cooki
Stir chocolate chips and Mini Eggs into the dough. Divide the batter into 3 tablespoon-sized balls, about 70-75g of dough per ball. If desired, stick additional chocolate chips, chunks of a chopped baking bar, and/or Mini Eggs into the tops of the cookies before baking.
Bake the cookies for 9-12 minutes until they are domed, slightly soft in the center, and firm around the edges. Allow the cookies to cool on the pan for about 10 minutes before transferring to a wire rack or plate.