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.

Bring to a boil and let it boil for 5 full minutes, stirring occasionally. Remove pan from heat and stir in vanilla extract.

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.

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.

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.

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.