Make the crust. Mix the cocoa, confectioners' sugar, and vanilla wafer crumbs in a small bowl. Mix in the melted butter. Press the mixture into the bottom of a greased 9-inch springform pan. Set aside.
Beat the cream cheese and butter. In a large bowl, combine the cream cheese and melted butter until smooth.
Add the cocoa. Mix in the cocoa until fully combined.
Add the eggs. While beating on low speed, add the eggs until just combined.
Add the milk and vanilla. Slowly mix in the sweetened condensed milk and vanilla extract until just combined.
Fold in the brownies. Gently fold in the crumbled brownies.
Assemble. Scoop the filling over the prepared crust.
Bake. Place the springform pan on a baking sheet and bake at 350°F for about 50–55 minutes, until the centre is almost set.
Cool. Transfer to a wire rack and let cool for 10 minutes. Loosen the cheesecake from the pan by carefully running a knife around the sides. Let cool for 1 hour more.
Chill. Refrigerate overnight.
Decorate and serve. Top with whipped cream and pecan halves if desired. Store leftovers in the refrigerator.