Preheat the oven to 350/177C. This recipe will make a three-layer cake using 8-inch cake pans and a two-layer cake using 9-inch cake pans. Line the pans with parchment paper and spray the sides with a baking spray.
Prepare the lemon cake: In a large mixing bowl, beat together softened butter, sugar and lemon extract for a few minutes until fluffy. Add the eggs and beat again until creamy and smooth, about 2 to 3 minutes. Next, pour in the milk but don't mix it in yet (I recommend using warm milk). In a separate bowl, combine all the dry ingredients: flour, baking powder and salt. Sift the dry ingredients into the cake batter and whisk in the dry ingredients and milk by hand to avoid over-mixing the batter. Add the lemon zest last and fold it in gently.
Divide the cake batter evenly between the prepared pans. Bake in preheated oven for approximately 30 minutes or until a toothpick inserted into the center comes out clean. Allow the cake layers to cool completely on a wire rack, then use a long, serrated knife to level off the tops. Place the cake layers into the freezer for faster cooling.
While the cake layers are baking, prepare the lemon curd. Juice approximately 4 to 5 large lemons over a fine mesh strainer set up over a glass bowl or large measuring cup. Double strain the juice for smoothest lemon curd. Set juice aside. In a small stainless steel pot, combine the eggs, egg yolks and sugar. Whisk vigorously for 3 to 4 minutes, until the mixture is thick and more pale in color; it should ribbon off the whisk. Add the lemon juice and whisk to combine.
Cook the lemon curd over medium-low heat, stirring constantly along the bottom of the pan with a rubber spatula. Cook for 4 to 5 minutes, until the curd thickens and starts to bubble lightly. Remove from heat immediately. Add the unsalted butter and vanilla and whisk until the butter is completely melted, then cover the lemon curd. Allow the lemon filling to cool slightly, then place into the refrigerator. Place into the freezer to cool faster.
Once the lemon curd and lemon cake layers are COMPLETELY cooled, prepare the mousse. Pour the chilled heavy cream into a large mixer bowl and add the sugar and lemon extract. Whisk on high speed for 3 to 4 minutes, until stiff peaks form. Reserve approximately 1 cup of the resulting whipped cream for garnishing the top. Add the chilled lemon curd to the whipped cream; reserve 1/2 cup of the lemon curd for spreading onto the cake layers. Whisk on medium speed for about 1 minute.
Prepare the gelatin: place the unflavored gelatin and water into a small bowl and stir to combine. Microwave the gelatin for 30 to 40 seconds, stirring often, until the gelatin is completely dissolved. With the mixer running on medium speed, pour the HOT gelatin into the mousse. Do not wait for the gelatin to cool; it will form clumps! Mix the mousse for another minute on medium speed until the mousse is uniform and smooth. Watch my video recipe to see how it's done.
To assemble the cake: line the sides of an 8-inch or 9-inch (depending on the size of your cake layers) with an acetate cake collar (link in post). A double layer of foil will work in a pinch. Make sure to the acetate collar overlaps a few inches to prevent the jello from pouring out! Spread a few tablespoons of the reserved lemon curd onto each cake layer. Place the first cake layer into the springform pan, then add 3 cups of the mousse. Spread it evenly to the edges with a spoon, then add the second layer. Repeat this process again, leveling the top layer of mousse as much as possible. Place the cake into the refrigerator to chill for 1 hour.
Prepare the lemon jello topping: in a measuring cup or small bowl, combine the flavored gelatin with 1/2 cup boiling water (this is half the recommended amount) and stir for a few minutes until the gelatin is completely dissolved. If needed, microwave the mixture for 30 seconds to dissolve completely. Add 1/2 cup ice cubes to cool the jello down. The mixture needs to be cool before pouring over the mousse. After the mousse cake has set for 1 hour, gently pour the jello over the top.
Refrigerate the lemon cake for an additional 6 to 8 hours, or overnight. When ready to enjoy, remove the springform and use a cake spatula to transfer the cake onto a stand. Remove the plastic acetate when ready to serve. Add dollops of the reserved whipped cream on top; I used an open French star tip #4. To prevent the cake from drying out, keep it wrapped in the plastic cake collar while refrigerated.