Preheat the oven to 350 F. Grease and line an 8 inch square pan (aluminum) with parchment paper.

Add the sugar to a large bowl and then zest the limes over it (careful to avoid getting the white pith). Rub the zest into the sugar with your fingers.

Add the softened butter to the bowl and beat to just mix. Add the dry ingredients then beat until mixed, when you can gather some of the dough in your palm, squeeze it and it keeps a ball shape, it’s done mixing.

Press the crust into the bottom of the prepared pan in an even layer. Use a measuring cup or offset spatula to press it into an even layer.

Set in the oven to bake for about 20 minutes, until golden all over the edges.

In a non-reactive pot, add the sugar and zest the limes over it. Rub between your fingers.

Juice the limes until you have the required amount (¾ cup) and pour into the pot. Separately, beat the eggs with a fork. Add the eggs to the pot then whisk the mixture together until all or most of the egg whites are broken up.

Set the pot over medium low heat and cook, stirring frequently with a silicone spatula, until thickened and covers the back of a spoon (on a thermometer it should read 170F). Remove from heat.

Slice the butter into a non-metal bowl and set a sieve over it. Pour the cooked filling through the sieve (you’ll be left with zest and bits of egg white - discard those). Stir the filing until the butter has melted and it’s smooth.

When the crust is done par-baking, take it out of the oven and pour in the filling. Smooth into an even layer. Return to the oven and bake until sides are set and there’s a slight jiggle in the center.

Set on the counter to cool to room temperature then set in the fridge to chill for 6 hours or up to 2 days.

When you’re ready to serve, sift a few tablespoons of powdered sugar over the bars, then slice and serve.

Leftovers keep in the fridge, in an airtight container (the powdered sugar will likely dissolve, you can add another layer of it if you like when you want to eat the leftovers).