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).

To fill cake: 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/frosting on your cake layers and pipe a border around the cake with buttercream.

For cupcakes: Cut a circle in a cooled cupcake, fill with raspberry filling, and 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.