Preheat oven to 250°F. Line two baking sheets with parchment paper. Using a 3-inch round cutter or bowl, trace 6 circles on each of the pieces of parchment paper. Make sure to space them out evenly. Flip the paper over so that the marks are on the underside. Set aside.
To make the meringue: In the bowl of an electric mixer, fitted with the whisk attachment, beat egg whites with the salt on medium-high speed until soft, glossy peaks form. While the mixer is running, add in the granulated sugar 1 tablespoon at a time. Beat the meringue on high until stiff and glossy peaks form, about 5 to 8 minutes more. Gently fold in the cornstarch, vinegar and vanilla until well combined. Fold in the sprinkles.
Using two spoons (or a pastry bag), scoop a mound of meringue in the center of each circle on the prepared baking sheets. Use the back of a spoon to create an indentation in the center of each meringue mound. Bake until firm and dry to the touch, but not browned, about 25 to 40 minutes. Turn off oven and let meringues cool for 1 hour, without opening the door. Once done, transfer to a wire rack to cool completely.
To make the pastry cream: In a medium saucepan, whisk together the sugar, cornstarch, and salt. Add in the milk and egg yolks and stir until well-combined. Set over moderate heat and bring to a boil. Let boil for 1 minute, whisking constantly. Remove from the heat and stir in the butter and vanilla until melted. Strain into a bowl through a fine mesh sieve, to remove any lumps. Cover with plastic wrap, pressing directly onto the surface of the custard to prevent a skin from forming. Refrigerate until chilled, at least 2 hours or overnight. Can be made up to 2 days in advance.
To make the whipped cream: In the bowl of a stand mixer (or you can use a bowl with a hand mixer) combine the heavy cream, powdered sugar and vanilla. Whisk on high until soft peaks form, about 3 to 5 minutes.
To assemble: Fill each meringue shell with the chilled pastry cream and a dollop of fresh whipped cream. Top with sprinkles and serve. Only assemble what you're going to eat right away. If you fill them all, they'll get soggy. You can make each component in advance. Just keep the custard and cream in the fridge and the meringue shells in an airtight container at room temperature. Assemble right before serving. Enjoy!