In a small saucepan, bring 2-3 inches of water to a simmer over medium heat.
Add the eggs and sugar to a small metal or glass bowl and set it over the saucepan, making sure the water doesn't touch the bottom of the bowl.
Whisk the eggs and sugar until smooth, then add the lemon juice and whisk to combine. Continue to cook, whisking until mixture is warmed through.
Add the butter, a few cubes at a time, whisking to incorporate. Continue whisking until butter melts, gradually adding more.
Continue whisking until it thickens slightly, forming a custard, and can coat the back of a spoon. This can take 15-25 minutes.
Once thickened, remove from heat and strain into a glass jar.
Enjoy warm or room temperature. Cover and store leftovers in the fridge for 2-3 weeks.