Place the Oreos in a food processor and pulse into fine crumbs.
Combine melted butter and cookie crumbs into a small bowl, and stir with a fork to mix well.
Press mixture into the bottom and 1-inch up the sides of a 9-inch springform pan.
Melt the chocolate in a double boiler. Pour over bottom of cookie crust and spread to the edges using an off-set spatula.
Place pan in the fridge while filling is prepared.
Pour the heavy cream into a mixing bowl and beat until soft peaks form. Transfer to a small bowl and store in the fridge until ready to use.
Clean mixing bowl, and place cream cheese and peanut butter in. Beat on medium speed until light and fluffy.
Reduce speed to low and gradually beat in the confectioner's sugar. Add the sweetened condensed milk, lemon juice, and vanilla extract.
Increase speed to medium and beat until all the ingredients are well combined and filling is smooth.
Stir in 1/3 of whipped cream into filling mixture. Then fold the rest of the whipped cream.
Pour the filling into the prepared springform pan. Drizzle the chocolate syrup on top and refrigerate for 3 hours or overnight before serving. Immediately before serving, scatter chopped and crumbled Reese's Peanut Butter Cups on top.