1- Whisk the cornstarch and water together until all the cornstarch has dissolved. (I just use a fork to mix—very easy.) Combine cornstarch mixture, raspberries (no need to thaw if using frozen), granulated sugar, and lemon juice in a medium saucepan set over medium heat. Using a silicone spatula, stir the mixture, mashing the raspberries as they begin to thaw and soften.
2- Bring to a boil and let it boil for 5 full minutes, stirring occasionally. Remove pan from heat and stir in vanilla extract.
3- Allow the raspberry filling to cool at room temperature for 10–15 minutes, then transfer it to a bowl or container and place it in the refrigerator for at least 4 hours and up to 1 week (the longer, the better). No need to cover it; but if refrigerating for longer than 4 hours, cover tightly. It will continue to thicken up as it chills. Raspberry filling must be completely cooled before using in your cake. If freezing, see Note below for instructions.
4- To fill cake: You can use the completely cooled and thickened raspberry filling to fill a 2-layer or 3-layer cake. You’ll have the most success with this raspberry filling if you partner it with some frosting filling; it really needs a buttercream base layer to keep the cake layers stable. Spread a thin layer of buttercream/frosting on your cake layers, to work as the sturdy base for your raspberry filling. Doesn’t need to be much; just a thin layer. You also need a buttercream “dam” around the cake layers to keep the jammy raspberry filling inside the cake layers. Spoon some buttercream into a piping bag fitted with a large round tip, such as Wilton 2A. Pipe a border around the cake. Then, spread the raspberry filling inside the buttercream border. An offset spatula is helpful for spreading. Assemble next cake layer on top, and repeat. For a 3-layer 8-inch or 9-inch cake, use a heaping 1/2 cup between each layer. For a 2-layer 8-inch or 9-inch cake, use about 3/4 cup filling; you’ll have some left over. See Note below for other cake sizes.
5- To fill cupcakes: With a sharp knife, cut a circle in a cooled cupcake, and remove the center, which will be roughly the shape of a cone. Using a small spoon, fill the middle of the cupcake with as much raspberry filling as you can. (Usually between 1–2 teaspoons.) Slice/tear off the pointed tip of the cone-shaped piece of cupcake, and gently press the round piece back on top of the filling. Repeat with remaining cupcakes. Makes enough filling to fill 2 dozen cupcakes. Or fill 1 dozen, and have some filling left over.
6- Cake or cupcakes filled with raspberry filling and topped with buttercream are typically fine covered at room temperature for 1 day. Cover and store in the refrigerator after that.