Pour the sugar into a medium sized pot and zest the limes over the sugar. Rub the sugar between your thumbs and fingers to release the oils.

Whisk this mixture with the eggs and a pinch of salt. Pour in the lime juice and whisk well.

Slice the butter and place in a bowl. Set a sieve over the bowl.

Set over medium low heat, whisking frequently as it thickens. Once it reaches 170 F or is thick enough to cover the back of a spoon, remove from heat.

Pour through the sieve to remove the egg bits and zest, pressing to get as much curd as possible. Stir until the curd is smooth and all the butter has melted.

In a heatproof bowl, melt the butter until it’s almost but not fully melted. Add the sugars and lime zest and whisk very well, for about 60 seconds.

Add the salt and vanilla and whisk. Now crack in the egg and whisk for another 60 seconds.

Add in the flour (if it’s lumpy sift it in) and the baking soda. Fold with a rubber spatula until dough is cohesive. Cover the bowl tightly with plastic wrap and set in the fridge.

Take the cookie dough out of the fridge a few hours before you plan to bake to allow it to soften. Scoop 1.5-2 tablespoons of cookie dough onto two prepared cookie pans (lined with parchment paper).

Repeat with all the dough balls. Sprinkle some of the ground crackers/cookies over the tops of the cookies, aiming to get most of the crumbs where the curd and cookie meet.

Set the trays in the freezer and freeze the cookies for at least 10 minutes before baking.

Preheat the oven to 350 F. The cookies need to be at least 1 inch apart (I bake 8 cookies per sheet) as they will spread.

Bake for 12-14 minutes, until the edges of the cookies are turning golden.