Preheat & Prep. To start, preheat the oven to 350 degrees F. Lightly grease a 9-inch tart pan with butter and set aside.
Make The Tart Crust. In a large mixing bowl, cream softened butter with brown sugar and granulated sugar for 2-3 minutes or until smooth and fluffy. Scrape down the sides of the bowl and add gluten-free flour and salt. Mix until the flour mixture is evenly coated in the butter mixture. The mixture may look a bit dry and shaggy, but if it holds together when pinched, you're good to go!
Bake The Crust. Use the bottom of a measuring cup or drinking glass to pat the crust mixture out into the prepared pan and up the sides. (Make sure to also go around the border of the crust with a finger to pack it down into an even layer.) Gently prick the crust 4-5 times with a fork and bake in preheated oven15-18 minutes, or until just golden brown. Set aside to cool slightly.
Mix Up The Lemon Filling. In a medium bowl, combine sweetened condensed milk, fresh lemon juice, egg yolks, vanilla extract, and lemon zest. Whisk until the lemon mixture is as smooth as possible. (There will still be flecks of zest, but you don't want any unmixed egg!)
Bake. Pour the tart filling into the baked tart shell and gently smooth into an even layer. Bake 12-14 minutes, or until the filling looks just set, with a subtle wiggle in the center.
Cool, Then Chill. Let the tart chill at room temperature 45-60 minutes, then transfer to the refrigerator and chill at least 3-4 hours, or until completely cold.
Decorate. Just before serving, decorate the tart with powdered sugar, fresh berries, whipped cream, lemon slices, edible flowers, or mint leaves, as desired.
Serve & Store. Cut into slices and enjoy! Store leftovers in an airtight container in the fridge 2-3 days or freeze up to 2 months.