Place a 5 quart pot filled with about 2 inches of water over medium-high heat and bring to a simmer.
Combine eggs, yolks, sugar, lime juice, olive oil and basil leaves in a large metal mixing bowl. Make sure the bowl is large enough to fit over the top of the pot without sinking into it. Or use a double-boiler set.
Place bowl over the simmering water and whisk constantly until the ingredients come together into a creamy custard. Do not leave unattended or stop whisking while the bowl is over the heat or you will curdle the eggs.
Once thick, remove the custard from the heat and slowly add the cold butter, stirring until completely mixed into the curd before adding more.
Remove the basil leaves from the curd and throw away.
Place curd in the fridge for a couple of hours until completely cool. Serve cold or use in tarts, etc.
Can be frozen for up to one year without harming.
Makes enough for 1 - 10 inch tart.