In a medium saucepan, whisk together eggs, sugar, lemon juice and lemon zest for a minute.
Turn on the heat to low and cook the mixture, whisking constantly, until the mixture reaches 170°F (75°C), 8-10 minutes. The mixture will be thick and it'll leave a trail if you run a finger on back of spoon dipped in curd.
Remove from heat. Place a mesh sieve over a bowl and run the curd through it. It makes the curd silky smooth!
Then cover the curd with plastic wrap, making sure it touches entire surface. This prevents forming an unappealing film over the curd. Let the curd cool for 10-15 minutes.
Add butter and stir until the butter is fully incorporated.
Transfer the lemon curd filling into a pastry bag, or a ziploc bag with one corner cut. Refrigerate it until ready to use. (TIP: I find it’s easier to pipe the filling, as it thickens quickly as it cools.)