Zest the limes, then slice them in half and juice them into a measuring cup. Use a small colander or a lemon juicer to keep out any seeds.
Place the egg yolks, cornstarch, and salt in a large bowl. Whisk well to combine and set aside. (You will be adding more to this later, so make sure it's a large enough bowl.)
Add your lime juice, granulated sugar, and lime zest to a saucepan and bring it to a simmer over medium heat, whisking to prevent burning.
When it has reached a simmer, scoop about 1 cup of the lime juice mixture and slowly add it into the egg yolk mixture while whisking continuously.
Add the tempered egg mixture back into the lime mixture while whisking constantly. Keep an eye on it and keep whisking until it thickens, if you walk away for even a minute, the eggs can curdle.
I cook my curd for about 2 minutes because I like thick lime curd. Use a thermometer to check the temperature of the lime curd. Removing the curd at 170 ºF (76 ºC)will yield a thinner consistency while removing it at 180 ºF (82ºC) will be thicker.
Add your butter in chunks to the lime curd and whisk it until the butter is melted and combined. Remove the lime curd from the heat. It will continue to thicken as it cools.
Add in your tequila and Grand Marnier (both optional) and stir until smooth.
Pour the finished lime curd into a heat-proof jar or bowl. Cover the curd with plastic wrap so that it is touching the surface of the curd without any air bubbles in between, this will prevent a skin from forming on the top of the curd. Refrigerate for minimum of 2 hours. Store it in the fridge for up to one week or freeze it for up to a year.