Preheat oven to 350°F.
Grease a 8in springform pan with butter and set aside.
Add oreos to a food processor until crushed into a fine crumb.
In a small bowl, add the melted butter and oreo crumbs, stirring to combine. Press this mixture into the bottom of the pan.
Bake for 8-10 minutes and allow to cool. Wrap the outside of the pan with foil and reduce oven temperature to 300.
In a large bowl with a mixer, beat the cream cheese, sugar and cornstarch until smooth.
Add in the eggs one at a time until mixed together.
Beat in the sour cream and vanilla until combined.
Divide the batter evenly into 3 bowls.
Stir the melted milk chocolate into one bowl and pour on top of the curst.
Whisk the pumpkin and pumpkin pie spice into another bowl. Carefully spoon this pumpkin cheesecake filling on top of the chocolate layer.
Stir in the melted semisweet chocolate into the last bowl of batter and carefully spoon this on top of the pumpkin layer.
Set springform pan in center of roasting pan. Place roasting pan in oven rack and add boiling water to come halfway up sides of springform pan.
Bake 1 1/2 to 1 3/4 hours until set, but center still jiggles slightly when shaken.
Turn off oven (leave door closed); let cake cool in oven 1 hour (if cake hasn’t pulled away from sides of pan, carefully run a thin knife around edge of pan to release cake). Remove foil. Cool cake in pan on a wire rack. Cover; refrigerate at least 4 hours or overnight.
Chocolate Glaze: Stir ingredients in a small saucepan over low heat until chocolate melts and mixture is smooth. Remove from heat; cool slightly.
Remove pan sides; place cake on serving plate. Spread glaze over top to edge (some may drip down side of cake). Refrigerate until glaze sets.
