For the ganache: Put the semisweet and bittersweet chocolate in a heatsafe bowl. Heat the cream until very hot in a saucepan and pour over the chocolate. Stir until melted and combined. Let cool.
For the milkshake: Put the milk, vanilla and chocolate ice cream and 1 cup of the ganache in a blender and blend until combined.
To assemble: Spoon some ganache into large mason jars and spread on the sides. Spread some ganache around the rim of the jars. Dip the rims in sprinkles, chocolate chips, marshmallows and/or candies, or press on pretzels. Add a scoop of ice cream to each glass and pour in the blended milkshake. Squirt whipped cream onto the top of each glass and add pieces of the bars or brownies. Now go crazy and decorate the top with more whipped cream, ice cream, candies, etc. Drizzle with caramel or strawberry sauce and top with a cherry.
Preheat the oven to 350 degrees F. Line a 9-inch square baking pan with aluminum foil and spray with nonstick spray.
Mix together the flour, oats, brown sugar, baking powder and salt in a bowl. Cut in the butter with a pastry cutter until the mixture resembles coarse crumbs. Sprinkle the mixture into the prepared pan and pat lightly to pack it slightly.
Bake until light golden brown on top and done in the middle, watching to make sure it doesn't burn, 30 to 35 minutes. Allow to cool for 15 minutes.
Spoon the dulce de leche into a microwave-safe bowl and microwave to just slightly soften it, about 45 seconds. Scoop it on top of the oatmeal base and use an offset spatula to spread it out to the edges and into an even layer. Sprinkle on a layer of peanuts so that they completely cover the caramel and use your hands to gently press them into the caramel. Sprinkle on the butterscotch chips and mini chocolate chips in generous layers.
Note: The warmth of the cookie base and caramel should slightly soften the butterscotch and chocolate chips. When that happens, use your hands to very gently press the chips just enough to anchor them together (but not enough to misshape them). If the pan isn't warm enough, pop it into the oven for 30 seconds or so and gently press the chips to anchor them together.
Chill the bars for 2 to 3 hours to make them easy to slice. Remove them from the pan, place on a cutting board and use a long serrated knife to cut into small squares (they're rich!). Serve cold or at room temperature.