Prepare an 8-inch square baking pan by lining with parchment paper.
For the bottom layer: In a large bowl mix together the graham cracker crumbs, coconut and finely chopped peanuts. Set aside.
In a saucepan, stir together the butter, sugar, beaten egg and cocoa. It will be a bit stiff, but it's important to start with the beaten egg in a cold pan and allow it to warm gently, to avoid scrambled eggs. Place saucepan on stove, over low heat and stir constantly for 5 minutes, until smooth and slightly thickened.
Add the butter/egg mixture to the graham cracker mixture and stir until well combined. Press this mixture evenly into the parchment lined 8-inch square pan and chill for 15 minutes. (*Tip: for firm, even layers, use a small object with a flat bottom like a drinking glass, to tamp down the bottom and middle layers).
For the middle layer: Beat together the peanut butter, butter, icing sugar, vanilla and milk until smooth. Taste test and if necessary, add salt, to taste. Spread this mixture over the chilled base. Smooth top and chill for 30 minutes or until firm.
For the top layer: Melt the chocolate and butter together in a double-boiler over hot water or in the microwave. Spread melted chocolate over the chilled middle layer. Sprinkle with a generous sprinkling of Fleur de Sel salt, if desired. Chill again until firm and then cut into squares.
*Tip: The easiest way to cut these, to avoid the chocolate cracking, is to run your knife under hot water for 30 seconds first, quickly dry off then use to cut the squares.
Be sure to read the “Ingredient and Cook's Notes" (above the recipe card!), where I share more detailedtips, variations and substitution suggestions for this recipe!