Whisk the cornstarch and water together until all the cornstarch has dissolved. 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). It will continue to thicken up as it chills. Raspberry filling must be completely cooled before using in your cake.

To fill cakes, you can use the completely cooled and thickened raspberry filling to fill a 2-layer or 3-layer cake. Spread a thin layer of buttercream on your cake layers, to serve as the sturdy base for your raspberry filling.

To fill cupcakes, cut a circle in a cooled cupcake, and remove the center. Fill with raspberry filling as much as you can. Replace the top.

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.