Shortcrust pastry:
In a food processor, cream the butter and icing sugar.
Add the almonds, salt, egg, and vanilla extract. Pulse until well combined.
Add the flour and pulse until a ball of dough forms.
Wrap the dough in plastic wrap and refrigerate for 2 hours.

Filling:
Mix the egg yolks with the sugar and brown sugar.
Add the cornstarch and mix with the water and key lime juice. Add the zests and mix well.
Put the mixture in a saucepan and bring to a boil, stirring constantly, until thickened.
Off the heat, add the butter and rum and mix well.

Assembly: