Cream the butter in the bowl of a stand mixer (or in a large bowl, using a hand mixer) until soft and whipped. Beat in the sugar and the lemon zest until light and fluffy. Add the eggs, one at a time. Whip in the lemon juice and continue to whip until it is incorporated (the mixture may look curdled at this point, which is normal).
Pour the curd into a saucepan and cook over medium-low heat, stirring frequently and whisking occasionally, for 10 minutes, or until the curd thickens and coats the back of a spoon. (If you have a candy thermometer and want to track the temperature, the curd will thicken at about 170°F / 77°C.) Don’t let the mixture boil, as this will create lumps. If it does boil, however, you can strain the finished curd through a fine-mesh sieve.
Allow to cool before making lemon bars, or you can store in the refrigerator for up to 2 weeks.
For lemon bars
Combine the gingersnap crumbs and granulated sugar in a bowl, and pour in the butter and 1 tablespoon water. Stir until the mixture is soft and well mixed. Press very firmly into a lightly greased 8-inch square baking dish and freeze while the filling is prepared.
Pour the lemon juice and 2 tablespoons water into a small saucepan and sprinkle the gelatin over the surface. Soften the gelatin for 5 minutes, then warm the pan over medium-low heat and stir until it is completely dissolved. Whisk it into the lemon curd. Spread the filling over the crust and refrigerate for 2 hours, or until set. Dust generously with powdered sugar just before serving and garnish with the lemon zest.