Place a baking sheet (parchment paper) in a buttered 9x13-inch pan so it covers the bottom and 2 sides (to facilitate the brownie removal). Butter the parchment paper or spray with baking spray and set aside.
In the bowl of a mixer fitted with the wire attachment, beat the eggs at medium speed until fluffy and light yellow, add the sugar and beat until combined. Add remaining ingredients, and mix to combine.
Pour half of the batter into the pan and top with Reese's eggs, pour the remaining batter.
Bake for 35-40 minutes. A toothpick inserted in the center of the brownie should come out with a few crumbs attached, but NO raw batter.
Remove from oven and set aside to cool without removing the brownie from the pan. Once cooled down completely start on the mousse.
Peanut Butter Mousse:
Chill a mixing bowl in the refrigerator or freezer for at least 15-20 minutes.
Remove bowl from refrigerator and using a mixer fitted with the wire attachment beat 1 cup of heavy cream until stiff peaks form. Set aside.
In a separate bowl, whisk on medium speed cream cheese, peanut butter and confectioner's sugar, reduce speed to low and add 1/2 cup of heavy cream, mix on medium low speed until fully combined. Stop to crape the sides of the bowl with a spatula.
Gently, using a spatula, manually fold in the whipped cream into the peanut butter mixture. Gently still until well blended.
Pour the peanut butter mousse on top of the completely cooled brownie pan and level the top layer. Place in the fridge overnight, or for at least 4-6 hours.
Before serving, remove from fridge, lift the mousse brownies from the pan using the parchment paper edges, dust with cocoa powder, run a knife thru cold water and cut into square bars.