Preheat oven to 350 degrees. Line 2 9" round pans with cooking spray and parchment paper.
For the cake: Add the cake mix, eggs, 2 tsp. lemon zest, and oil to mixing bowl. Add 1/2 tsp. of lemon extract if using white cake mix. Mix on medium for 2 minutes.
Divide batter evenly between pans. Mine divides into 450g of batter each. Bake for 20-25 minutes. A toothpick should come out clean.
For the whipped cream filling: First, in a small bowl bloom the gelatin by adding the gelatin to the lemon juice. Allow to sit 5 minutes. Then, warm in the microwave on half power for 20-25 seconds or until completely dissolved. Allow to cool. Whip the whipping cream on high until stiff peaks form. While the mixer is on, drizzle the gelatin mixture onto the side of the bowl. Set aside in the fridge.
Mix cream cheese, 2 tsp. lemon zest, 1/2 tsp. lemon extract and 2 cups powdered sugar with a mixer until combined thoroughly. Add the whipped cream and fold in gently. Store in fridge until ready to assemble cake.
For the crumb topping: In a medium bowl, add 1/2 cup powdered sugar, flour, butter and lemon extract. Using forks or your fingertips, crumble and mix until the butter is the size of a pea or smaller.
To assemble: Level the two cakes by cutting off the dome. Place the bottom cake on a platter. Add about three-fourths of the cream mixture. Top with the other cake. Add the rest of the cream mixture to the top and fill in the sides if there is a gap. Sprinkle the crumb topping. I usually only use some of the crumb topping and run out of butter.
Put cake in the fridge for 3 hours to 3 days. Covered and kept cold. Serve cold.
Notes
Recipe adapted from Todd Wilbur's Top Secret Recipes 2.