Several hours before you're ready to assemble the sundae, portion out 12 scoops of each type of ice cream--vanilla, mint chip and strawberry--onto baking sheets (36 scoops total) and freeze them. This will make it much easier to handle.
Line the guttering with heavy-duty aluminum foil.
To assemble, place a layer of Dark Chocolate Brownie Bites in the bottom of the guttering. Top with the ice cream scoops and scatter over more brownie bites. Drizzle with Strawberry Sauce and Hot Fudge Sauce. Sprinkle with rainbow sprinkles. Squirt mounds of whipped cream across the sundae and top each mound with a cherry.
Dark Chocolate Brownie Bites:
Preheat the oven to 350 degrees F. Grease an 18-by-13-inch pan (half sheet pan) with cooking spray.
In a medium-large saucepan, heat the butter and unsweetened chocolate over low heat, whisking occasionally, until smooth and melted. Sprinkle in the cocoa powder and whisk to combine. Remove the pan from the heat and allow to cool for about 5 minutes.
Stir the sugar and vanilla into the chocolate mixture until just combined. One at a time, stir in the eggs. Gently stir in the flour until halfway incorporated. Add the chocolate chips and stir just until combined.
Pour the batter (it will be very thick!) into the prepared pan, spreading to even out the top. Bake for 30 minutes, then check the brownies with a toothpick. If they are overly gooey/messy, return to the oven for 5 to 10 more minutes (just be careful not to burn the edges). Allow to cool completely (important!), then cut into small squares.
Place the strawberries, sugar, vanilla and lemon juice in a medium saucepan over medium-high heat. Bring to a gentle boil, stirring constantly, and cook until the strawberries are very soft, a good 5 minutes. Turn off the heat and use a potato masher to completely smush them to smithereens. Pour the mixture into a fine-mesh strainer placed over a bowl. Use a spoon to stir the fruit so the liquid is forced through. Continue until almost all of the liquid is in the bowl. Set the pulp aside.
Skim as much foam off the sauce as you can. Pour the skimmed sauce back into the same saucepan, add 1/4 cup of the pulp back in and boil again for 3 minutes over medium-high heat. Skim off any more foam that has formed on the surface, then allow the sauce to cool slightly before pouring it into a pitcher or jar. Refrigerate it to thicken and serve it cold.
Hot Fudge Sauce:
Combine the sugar, cream, cocoa and butter in a small saucepot over medium heat and cook, stirring, until the butter melts and everything is combined, a few minutes.
Transfer to a mason jar and store in the refrigerator. It will harden as it cools so when ready to use, simply reheat in a saucepan over medium-low heat.