make the cookie base
Place a rack in the middle of the oven and preheat it to 180 °C / 350 °F. Combine ground Graham cracker cookies, sugar, and cocoa in a small bowl. Add the melted butter and milk, and stir to combine. Transfer the mixture to a 30 cm (12-inch) Removable Bottom Tart Pan. Press the homemade cookie dough onto the bottom and up the side of the cake or pie pan. Transfer to the preheated oven on the middle rack. Bake for 10 minutes at 180 °C / 350 °F.
Place a pan over medium heat. Add the butter and let it melt. Add the condensed milk and stir to combine. Sift the cocoa powder and add to the pan. Reduce the heat and cook for 10 - 12 minutes, stirring regularly for the mixture to thicken. The cream is ready when we run the spatula at the bottom of the pan, and the mixture needs about 3 - 4 seconds to cover the surface again. Remove from the heat and add in the chopped chocolate. Stir until the chocolate melts.
Immediately pour the Brigadeiro mixture into the slightly cooled cookie crust. Arrange the chocolate sprinkles on top and set aside. Set at room temperature for 2 - 3 hours, or place in the fridge overnight. If you're chilling the tart in the refrigerator, bring it to room temperature 30 minutes before serving.