Oreo Crumble: Preheat oven to 350°F. Place the cookies in a large ziploc bag and use a rolling pin to crush them. Melt butter in a medium bowl on HIGH power, then add the cookies to the melted butter. Stir until all the cookies are moistened with the butter. Bake for 10 minutes on a rimmed baking sheet. Let cool completely before making peanut butter mousse.
Peanut Butter Mousse: In the bowl of an electric mixer fitted with the whisk attachment (or using a hand mixer) beat heavy cream and 1/4 cup powdered sugar until stiff peaks form. Transfer to a bowl and refrigerate. Wipe out the mixer bowl (no need to wash it).
Place cream cheese and peanut butter in the mixer bowl and blend with the paddle attachment until smooth. Slowly add the remaining 1 1/4 cups powdered sugar. Add the milk and vanilla. Fold in the whipped cream.
Assemble the Trifles: Place 1-2 tablespoons of crumble in the bottom of your serving dishes, deepening on the size of the dish. Spoon a layer of the peanut butter mousse over the crumble. Repeat with remaining crumble and mousse. Top with half of a peanut butter cup for garnish.
Refrigerate at least 15 minutes before serving. (Desserts can be made up to 4 hours head.)
Note: these are very rich. Consider using small serving dishes or 2 ounce shot glasses instead of bowls.
Tip: substitute Cool Whip (regular, lite, or fat-free) for the heavy cream (skip whipping the cream and fold in the Cool whip instead).