In a heatproof glass bowl, whisk together 1/2 cup granulated sugar, 1/4 cup freshly squeezed lemon juice, zest from 1 lemon, and 4 large egg yolks. Set the bowl over a pot of simmering water over medium heat to make a double boiler, making sure that the bottom of the bowl isn't touching the water. Whisk occasionally, cooking the mixture until it thickens considerably and a candy thermometer placed in the middle of the mixture reads 172 degrees (F).