This is a multi-stage process requiring significant freezing time. Plan ahead!

Stage 1: Prepare Base Layers (Ideally Day 1 or 2)

1. Prep Pans: Line four 8-inch round pans thoroughly with plastic wrap, leaving overhangs. Place them in the freezer to chill.
2. Make Crust: Mix chocolate cookie crumbs and melted butter. Press firmly into the bottom of one of the chilled, lined pans. Freeze for 15 minutes until firm.
3. Layer Vanilla: Remove the pan with the crust. Carefully press/spread the slightly softened vanilla ice cream evenly on top of the crust, smoothing the top. Cover tightly with plastic wrap and return to the freezer.
4. Layer Other Ice Creams: Working one flavor at a time, press/spread the slightly softened chocolate ice cream into a second chilled, lined pan. Press/spread the slightly softened birthday cake ice cream into a third chilled, lined pan. Cover both tightly with plastic wrap and return to the freezer.
5. Ensure Brownie Layer is Ready: Have your pre-baked, cooled 8-inch round brownie layer ready. (If baking yourself, do this well in advance).
6. Freeze Solid: Freeze all three ice cream layers (vanilla/crust, chocolate, birthday cake) and the brownie layer (if just baked) for at least 4 hours, or preferably overnight, until completely solid.

Stage 2: Stack the Base (Ideally Day 2 or 3)

7. Work Quickly! Remove the frozen layers from the freezer. Using the plastic wrap overhangs, lift the vanilla/crust layer out and place it (crust-side down) onto your final serving platter or cake stand (ensure it fits in your freezer!). Remove plastic wrap.
8. Add Chocolate Layer: Unwrap the solid chocolate ice cream layer and place it directly on top of the vanilla layer. Press gently.
9. Add Brownie Layer: Place the 8-inch round brownie layer on top of the chocolate ice cream. Press firmly to adhere.
10. Add Birthday Cake Layer: Unwrap the solid birthday cake ice cream layer and place it on top of the brownie layer. Press gently.
11. Wrap and Freeze: Immediately wrap the entire stack tightly and carefully in plastic wrap. Return the stacked base to the freezer until completely solid again (at least 2-4 hours).

Stage 3: Create the Crown Topping (Ideally Day 3 or 4)

12. Prep Springform: Line an 8-inch springform pan with plastic wrap (bottom and sides). Place it on ice or in the freezer to chill thoroughly.
13. Prep Ice Cream Pints: Open all the Häagen-Dazs pints. Keep them on ice or work very quickly.
14. Assemble Crown: Scoop small balls of the different Häagen-Dazs ice cream flavors and press them into the chilled, lined springform pan, alternating with the mini meringues. Build upwards, mounding the scoops into a little mountain shape. Work as quickly as you can to prevent melting.
15. Freeze Crown Solid: Cover the springform pan tightly with plastic wrap. Place in the freezer and freeze for at least 4 hours, or preferably overnight, until it’s one solid lump of ice cream and meringues.

Stage 4: Make Stabilized Frosting & Final Assembly (Party Day!)

16. Make Stabilized Whipped Cream:

    • Bloom Gelatin: Soften the gelatin sheet in a small bowl of cold water OR sprinkle powdered gelatin over 1 tablespoon of cold water in a small microwave-safe bowl and let stand 1-5 minutes to bloom.
    • Dissolve Gelatin: Gently warm the bloomed gelatin until dissolved (microwave bloomed powder for 5-10 seconds, or gently heat softened sheet in 1 tbsp warm water). Do not boil. Let cool slightly but remain liquid.
    • Whip Cream: Combine the cold heavy cream and cane sugar (and vanilla, if using) in the chilled bowl of a stand mixer. Beat on medium speed for 30 seconds.
    • Add Gelatin: With the mixer running on low, slowly drizzle in the slightly cooled, liquid gelatin mixture.
    • Whip to Stiff Peaks: Increase mixer speed to high and whip until the cream is stiff and holds its shape, but is not grainy. Do not overwhip. Chill briefly if not using immediately (up to 1 hour).

17. Frost the Base Cake: Spread a little whipped frosting onto your final chilled cake stand (this helps anchor the cake). Unwrap the frozen stacked ice cream ‘base’. Carefully center it on the cake stand. Working quickly, frost the top and sides generously with the stabilized whipped cream frosting.

18. Add the Crown: Unwrap the frozen ice cream cake ‘topping’ crown. Carefully lift it from the springform pan (using plastic wrap) and place it centrally on top of the frosted base like a crown. Press gently.

19. Final Decoration & Freeze: Place any remaining whipped cream in a piping bag fitted with a large swirl or star tip. Pipe a decorative border around the base of the cake stack. Sprinkle the top crown lightly with mini chocolate chips. Immediately return the entire assembled cake to the freezer for a few more hours to ensure everything is solid and the frosting is firm.

20. Serve: Remove from freezer 10-15 minutes before serving to soften slightly. Slice carefully with a large, sharp knife dipped in hot water and wiped clean between cuts. Enjoy your masterpiece!