Preheat oven to 325ºF and line 15 standards-size muffin cups with paper liners (or use free-standing liners).
In a small bowl, whisk together the graham cracker crumbs and brown sugar. Add melted butter and mix in until fully incorporated.
Divide mixture between liners (about 1 tablespoon per liner) and use a shot glass to press into the bottom of each liner in a flat layer.
Bake crust in preheated oven for 8 minutes. Allow to cool while prepping cupcake batter.
Increase oven temperature to 350ºF.
In a medium bowl, whisk together the flour, baking powder, xanthan gum, and salt.
In a separate bowl, whisk together the butter, sugar, lemon juice, vanilla extract, and lemon zest. Add the eggs and beat in until smoothly incorporated.
Combine wet ingredients with dry and fold in until evenly combined. Divide batter between liners (about 1 heaping tablespoon each) and smooth out surfaces.
Bake cupcakes in prehated oven for 18-20 minutes, until a toothpick inserted into the center comes out clean. Transfer to a wire rack to cool completely.
Once cool top each cupcake with 1 tablespoon lemon curd, smoothing out surface with an offset spatula.
In the bowl of a stand mixer fitted with the wire whip attachment, beat eggs on medium-high until soft peaks form. Add 1 tablespoon sugar and beat in briefly to incorporate.
In a small saucepan, whisk together remaining 5 1/4 ounces (3/4 cup) granulated sugar with the water and corn syrup. Set over medium heat and stir constantly until sugar has dissolved. Bring to a boil and maintain, without stirring, until syrup reaches 230ºF as measured by an instant-read thermometer**.
With mixer on low speed, slow pour sugar syrup along the side of the bowl into egg whites. Once all has been added, increase speed to medium-high and continue to beat until both the bowl and the meringue are no longer warm; meringue should be fairly stiff and glossy.
Transfer frosting to a large piping bag fitted with a large open start tip (I used Ateco #827 and pipe onto cupcakes. Use a kitchen torch to carefully toast meringue.
Cupcakes are best served at room temperature. Store any leftovers in an airtight container in the refrigerator.
*Preferably light corn syrup, but both light and dark will work.
**Having a reliable thermometer is crucial for making meringue. I highly recommend the Thermapen as it’s essentially the best of the best when it comes to home kitchen thermometers!