Add the cornstarch to the water and stir so it melts. Then add everything into a medium size pot and bring to a boil until thickens. Use a whisk to mix the berries up. Let it simmer for 15 minutes stirring often. Strain the mixture through a sieve.

Make a hole in the top of the cupcake and then fill with raspberry filling. Add 2 tablespoons of the berry sauce to the frosting. Do not stir! Just add it to the piping bag and pipe. The frosting will come out with a nice raspberry swirl!