In a medium heatproof bowl, whisk together the eggs, granulated sugar, fresh lemon juice, and lemon zest until fully combined.
Place the bowl over a saucepan of simmering water, ensuring the bottom of the bowl does not touch the water (double boiler method).
Cook the mixture, whisking constantly, until it thickens and reaches about 170°F (77°C) on a thermometer, approximately 10 to 15 minutes.
Remove the bowl from heat and immediately whisk in the unsalted butter pieces one at a time until fully melted and incorporated, creating a smooth curd.
Pour the lemon curd into a clean jar or container and allow it to cool to room temperature.
Once cooled, cover and refrigerate for at least 2 hours before using as a muffin filling or topping.