You can try lemon curd.

5 egg yolks
1 cup sugar
1/3 cup fresh lemon juice (3-4 lemons)
1 tablespoon lemon zest
1 stick butter, cut into pieces

Add 1 inch of water in a saucepan, bring to simmer. Combine egg yolks
and sugar in a metal bowl and whisk until smooth. Add juice and zest to
egg mixture and whisk. Reduce heat to low and place bowl on top of
saucepan, without the bowl touching the water. Whisk until thickened.
It will be light yellow and it coats the back of a spoon (around 8
minutes). Remove from heat and stir in butter, one piece at a time,
allowing the butter to melt before adding more butter. Cover with
plastic wrap and refrigerate until use.

You have to handcuff me to keep me from eating the whole bowl. Or maybe
I just like the handcuffs...

Best of luck with your cake.

Becca