Mix in the vanilla extract and lemon extract, lemon zest and lime zest.
Gradually add in the flour and salt and mix until combined.
Mix in the heavy whipping cream and 7UP until well combined and batter is fluffy.
pour into 4 inch round cake pans - (it make 5 using a 3 inch tall pan) or in a 9x13 pan (you'll have to cut the circles out later)
bake until the center is set - the 9x13 pan will take about 40 mins - for the 4x3 round pans - beause there is so much batter and height its a little tricky - I baked it at 425 in a water bath (4 inch pans placed inside another pan with water in it) for 40 mins, then pulled the pans out of the waterbath and baked at 410 for another 30 mins - to try to cook the inside and outside evenly
let cool or freeze - level the top and bottom and cut each cake in half - I also ended up cutting the outer edge to fit into the chocolate shell - you'll need SIX 4 inch circles each about 1 1/4 inch tall
brush with the 7UP syrup for an even stronger flavor
take the remaining soda (about 7 1/2 cups) and boil in a saucepan uncovered
it will boil down and get thick, pull off the stove when it's down to about 1 1/4 C
in a saucepan bring to a simmer the lemon zest, lime zest and 1/4 C syrup
in another bowl beat the eggs and sugar
while stirring pour the hot syrup mixture into the eggs to temper them - them pour the egg mixture back into the sauce pan and cook over medium/low heat until thick.
strain into a clean bowl and add the butter and pinch of salt - stir until the butter is melted
cover with plastic and let cool in fridge
pour the gelatin into the water and let bloom, then microwave 30 seconds until melted - set aside and let cool
beat the cream, sugar and vanilla until soft peaks form, while beating pour the gelatin mixture in
beat until stiff peaks form
stir into the lemon/lime curd and refrigerate
beat the butter until fluffy
add the 7UP syrup and beat until incorporated.
add half the powdered sugar and the salt and beat - adding the rest of the powdered sugar until your happy with the thickness - not too thick as its for frosting not decorating