Prep Tart Crust - In a stand mixer fitted with the paddle attachment, beat the softened butter and powdered sugar on medium speed until light and fluffy. Add the vanilla extract and salt, mixing until combined. Scrape down the sides of the bowl with a rubber spatula. Add the egg and mix until fully incorporated.
Add the Dry Ingredients - To form your pie crust, add the almond flour first, followed by the all-purpose flour. Mix on low speed just until incorporated. Be careful not to overmix, as too much air can make the crust tough. Once combined, use a bowl scraper to remove the dough and form it into a flat disc. Wrap tightly in plastic wrap and chill for at least 2 hours.
Form Tart Crust - Once chilled, lightly dust your surface with powdered sugar and roll the dough to 1/8-inch thickness, rotating frequently to prevent sticking. Spray your tart pan with non-stick spray and gently press the dough into the pan, trimming any excess. For mini tarts: Dock the bottoms with a fork. For full tarts: Line with parchment and add pie weights.
Bake and Cool - Bake at 325°F (163°C) for about 15 minutes, or until the edges are golden brown with a pale center. Remove from the pan while still warm and allow to cool on a wire rack.
Prep Key Lime Curd - While tarts are baking, you can bloom your gelatin to prep for your key lime curd. To start, place the butter into a mixing bowl (glass or metal recommended). Submerge the gelatin sheet in ice-cold water and allow it to soften for a few minutes, this process is called blooming.
Combine Egg Yolks, Sugar, and Key Lime Juice - In a bowl, whisk together the egg yolks and sugar immediately to prevent the sugar from burning the yolks. Slowly whisk in the fresh key lime juice until fully combined. Then you're going to place the mixture over a double boiler on medium-low heat. Cook gently, whisking constantly, until the curd thickens and coats the back of a spoon.
Strain, Finish, and Chill - Pour the hot curd through a fine mesh strainer into the bowl with the butter and bloomed gelatin. This removes any cooked egg bits and ensures a smooth texture. Whisk until fully combined and silky. Optionally, add a touch of green food coloring for a soft tint. Press plastic wrap directly onto the surface to prevent a skin from forming and allow the curd to cool completely before using. Set aside.
Fill and Set the Tart - Pour or pipe the cooled lime curd into the cooled tart shell and smooth the top with the back of a spoon or an offset spatula. Cover and refrigerate for as you prep the Swiss meringue.
Make Swiss Meringue - You will need to set up a double boiler to prep the egg whites for this meringue. Ensure egg whites are room temperature. Fill a saucepan with about 1 inch of water and bring to a gentle simmer. Place a heat-safe bowl over the pan, ensuring it does not touch the water.
Heat Egg Whites and Sugar - Inside the bowl, add the egg whites and sugar to the bowl. Whisk constantly until the sugar fully dissolves and the mixture feels smooth (not grainy) when rubbed between your fingers. The mixture should look glossy and feel warm.
Whip the Meringue - Remove the bowl from heat and dry the bottom. Transfer to a stand mixer fitted with a whisk attachment. Beat on medium speed, then increase to medium-high until stiff peaks form. Add vanilla bean paste if desired. Continue whipping until the bowl cools to room temperature and the meringue becomes light, fluffy, and stable. Transfer to a piping bag to use immediately.
Assemble and Serve - Once the tart is fully set, top with Swiss meringue as desired. Garnish with lime zest, lime slices, or elegant finishing touches for presentation. Serve chilled and enjoy this bright, citrus-forward dessert!