Place all ingredients into a small saucepan. Whisk to combine. Place over medium heat and let come to a slow boil, whisking constantly.
Continue to boil gently for 3-4 minutes, stirring constantly. Take off the heat, let cool. Use in cupcakes or store in an airtight container in the fridge.
Preheat oven to 350 degrees. Line a cupcake tin with cupcake liners, set aside.
In a food processor, place the sugar and pulse until very fine, about 2 minutes. Sift half of the sugar with the flour and salt, set aside.
In a large bowl with a hand mixer or a stand mixer, add the egg whites, add in the water, vanilla and cream of tartar. Mix on low until fully combined. Gradually add in the remaining sugar until incorporated. Place mixer on medium speed until egg whites form medium peaks.
Start to add in the flour mixture a little at a time by sifting it onto the surface of the egg mixture and folding it in in very small batches by hand.
Fill the cupcake liners almost all the way to the top, about 3/4 full. Bake for 16 to 18 minutes or until a toothpick inserted into the middle comes out clean. Let cool for 5 minutes in the tin, then transfer to a wire rack to cool completely.
In a large bowl, add the heavy cream. Using a hand mixer or stand mixer with the whisk attachment, whisk on low until bubbles start to form. Add in the powdered sugar one tablespoon at a time, whisking between additions. Add the cornstarch and vanilla once the cream forms soft peaks.
Continue to whip on medium speed until the cream will stand up on the whisk and will not move. Do not over beat or you will have butter. Add the lemon curd, and fold in by hand until combined.
Only assembly once the cupcakes are completely cooled. Add lemon curd to a piping bag with a small round tip. Gently press into the middle of the cupcake about 1/2 inch down. Squeeze some curd into the cupcake, only about a teaspoons worth. Continue with all the cupcakes.
Place whipped cream into a piping bag or frost however you wish. Add mint leaves and sprinkles for garnish, if desired.