Bake the cake according to the instructions for a 9x13 sheet cake.
Once the cake has baked and cooled level it using a knife or a cake leveler. Then use a 3-inch round cookie cutter to cut out as many circles as you can (you should get at least 6).
Place these cake rounds in an airtight container and place in the freezer for at least 1 hour or overnight (you can make these up to a week ahead of time).
In a small saucepan combine raspberries, sugar, and lemon juice. Cook over low heat until the sugar has dissolved.
Pour the raspberry puree into a fine metal sieve, pressing on it to strain the seeds out of the raspberry puree. Set to the side and let cool to room temperature.
In a small bowl add 2 teaspoons of gelatin and stir in 2 tablespoons of water. Stir until the gelatin is dissolved and set to the side to let it firm up.
Pour the heavy whipped cream into the bowl of your mixer and set it to high, whipping the heavy cream into stiff peaks.
Place the gelatin in the microwave and cook for 10 seconds so the gelatin softens back into a pourable consistency.
Add the gelatin to the cooled raspberry puree and stir to combine.
Gently fold the raspberry puree into the whipped cream until it is fully combined.
Remove cake rounds from the freezer.
Wrap an acetate cake strip around one of the cake rounds and cut to fit. Use this piece to measure and cut cake collars for each cake round.
Cut each cake round in half to make two thin layers. Remove the top halves and set to the side.
Wrap the acetate cake collars around the bottom half of each round using a piece of tape to secure it. Make sure the cake collar creates a cylinder that is the same width at the top as the bottom. Repeat for the bottom layer of each cake round.
Place the cake rounds wrapped with the acetate cake collar on a baking sheet that will fit in the freezer.
Fill a piping bag with your raspberry mousse and snip off the end.
Pipe the raspberry mousse onto the bottom layer making a layer that is about 1 inch thick.
Place the top cake layer on top of the mousse and press down lightly.
Pipe a second layer of raspberry mousse on top trying to make it level.
Repeat for all of the cake rounds.
Place the raspberry mousse cakes in the freezer and leave for at least 30 minutes. The longer the better.
When ready remove from the freezer and gently remove the cake collar.
The cakes will thaw quickly so you can wait a few moments and serve or you can place them in an airtight container and put them back in the freezer for 2-3 days.
Garnish with a fresh raspberry when ready to serve.
Note: If you would like the raspberry mousse to be a little more pink you can add a drop of pink food coloring.