• In a medium saucepan, whisk together cornstarch with water until dissolved. Add raspberries (frozen is fine), lemon juice, and granulated sugar.
• Heat them on medium and stir with a rubber spatula until raspberries soften. Mash gently and boil the mixture, then simmer for 5 minutes. Remove from the heat and whisk in some vanilla extract.
• Let the filling cool for 10 to 15 minutes at room temperature, then refrigerate for at least 4 hours. For longer periods, seal the container tightly.

Filling Cakes:

• Spread thin buttercream or frosting on cake layers to stabilize. Pipe a border of buttercream around the edges.
• Spoon raspberry filling inside this buttercream border, stacking each layer of cake until complete. Use half a cup of filling between layers for a 3-layered cake or ¾ cup for a 2-layered one. Leftover filling can be used for cupcakes or toast.

Filling Cupcakes:

• Cool cupcakes and use a knife to cut a small circle in the center, removing the piece to create a well.
• Fill the well with 1 to 2 teaspoons of raspberry filling. Trim the removed piece and place it back on the filling.
• Repeat with your remaining cupcakes. The recipe yields enough filling for three dozen cupcakes (or fewer if desired; any leftover filling can be saved for later use).