Prepare the shortbread crust first. Place the softened butter and sugar into a mixer bowl and beat on medium speed for 3 to 4 minutes until creamy and fluffy. Add in the egg and salt and continue mixing for 1 minute, until the mixture is creamy. Add in the flour and mix on low speed for 1 minute, or until a soft dough comes together. Turn the dough out onto a floured work surface and gently bring the dough together. Using a well-floured rolling pin, roll the dough out to 1/8th-inch thickness. You don't want it to be too thick or too thin. Next, use a 2-inch round cookie cutter and cut out circles for each tart.
Drop the shaped dough into mini cupcakes tins or tartlet tins and shape the dough with back of small spoon or your fingertips. Once the whole pan has been filled, place the pan into the refrigerator to chill for at least 1 hour. Once chilled, preheat the oven to 350F/177C and bake for 20 to 22 minutes, until the crust is just light golden brown. Allow the tartlet shells to cool in the pan for about 20 minutes, then remove onto a wire rack to cool completely.
Making the Lemon Curd:
Next, prepare the lemon curd; this can be prepared the day before. Juice approximately 4 to 5 large lemons over a fine mesh strainer set up over a glass bowl or large measuring cup. Double strain the juice for smoothest lemon curd. Set juice aside. In a separate glass mixing bowl, combine the eggs, egg yolks and sugar. Whisk vigorously for 3 to 4 minutes, until the mixture is thick and more pale in color. It should ribbon off the whisk. Pour the mixture into a stainless steel pot and add the lemon juice; whisk to combine. Cook the lemon curd over medium-low heat, stirring constantly along the bottom of the pan. Cook for 4 to 5 minutes, until the curd thickens and starts to bubble lightly. Remove from heat immediately. Add the unsalted butter and vanilla and whisk until the butter is completely melted. Allow the lemon curd to cool completely in the refrigerator.
Making the Raspberry Sauce:
Next, prepare the raspberry sauce. Place the berries and sugar into a medium saucepan and using a potato masher, crush the berries into a pulp. Bring the sauce to a simmer over medium heat and continue cooking, stirring occasionally, for about 10 minutes. Once cooked, pour the raspberry sauce into a fine mesh strainer arranged over a large bowl. Use the back of a spoon to press the sauce the sauce through the strainer until just the seeds remain, discarding the seeds. Watch my video recipe to see how it's done.
Making the Meringue:
For the meringue, place the egg whites into a very clean mixer bowl (wash with degreasing kitchen soap first!) and whisk on medium speed for 2 to 3 minutes, just until soft peaks form. In a small saucepan, add the sugar and water but do not stir! Attach a candy thermometer to the side of the saucepan and cook the sugar syrup over medium-high heat for 5 to 7 minutes, until the syrup reaches 240F/115C. With the mixer running on medium speed, slowly pour in the hot syrup, avoiding the whisk; do this immediately after the sugar syrup reaches 240F. Add in the lemon juice and vanilla and continue whisking on high speed for 6 to 8 minutes, until stiff and glossy peaks form; the meringue will begin to cool as it's mixed. Transfer the meringue into a pastry bag tipped with a star tip; I used Ateco French star tip size #8.
Assembling the Tarts:
To assemble the tartlets, fill them almost to the top with the lemon curd, then top with a bit of raspberry sauce. I recommend transfering the curd and sauce into a pastry bag or ziplock bag and piping into the tart shells. Next, top with a dollop of meringue. For garnishing, I like to use a kitchen torch and torch the meringue. You can also drizzle the meringue with more raspberry sauce. Tartlets can be kept at room temperature for 24 hours, then refrigerate.