Whisk the cornstarch and water together until the cornstarch dissolves.

In a medium saucepan, mix the cornstarch-water mixture with raspberries, granulated sugar, and lemon juice.

Place the saucepan over medium heat and stir the mixture, mashing the raspberries as they soften.

Allow the mixture to come to a boil, then cook for 5 minutes, stirring occasionally.

Remove the pan from heat and stir in the vanilla extract.

Let the raspberry filling cool at room temperature for about 10–15 minutes.

Transfer the cooled filling to a bowl or container and refrigerate for at least 4 hours.

For cakes, spread a thin layer of buttercream on the cake layers, then pipe a buttercream border around the edges.

Spoon the raspberry filling inside the border and use an offset spatula to spread it evenly.

Repeat with additional layers as needed.

For cupcakes, cut out a small cone from the center, fill with raspberry filling, and replace the piece.