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. 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. It will continue to thicken up as it chills. Raspberry filling must be completely cooled before using in your cake.

To fill cake: Use the completely cooled raspberry filling to fill a 2-layer or 3-layer cake. Spread a thin layer of buttercream on your cake layers to work as a sturdy base for your raspberry filling. Pipe a border around the cake and spread raspberry filling inside the buttercream border.

To fill cupcakes: Cut a circle in a cooled cupcake, remove the center, and fill with raspberry filling as much as you can. Press the round piece back on top.