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 place it in the refrigerator for at least 4 hours and up to 1 week.

To fill cake: Use the cooled and thickened raspberry filling to fill a 2-layer or 3-layer cake, spreading a thin layer of buttercream on the layers as a base.

To fill cupcakes: Cut a circle in a cooled cupcake, fill with as much raspberry filling as possible, then press the cone-shaped piece back on.

Filled cakes or cupcakes can be covered at room temperature for 1 day. Cover and store in the refrigerator afterward.