Whisk the cornstarch and water together until all the cornstarch has dissolved. Combine cornstarch mixture, raspberries, granulated sugar, and lemon juice in a medium saucepan set over medium heat. 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 refrigerate for at least 4 hours.

To fill cake, spread a thin layer of buttercream on your cake layers, then pipe a border around the layers and spread the raspberry filling inside the buttercream border.

To fill cupcakes, cut a circle in a cooled cupcake, fill with raspberry filling, and replace the top.

Cake or cupcakes filled with raspberry filling can be covered at room temperature for 1 day, then store in the refrigerator.