Bake cupcakes according to the package directions and let cool completely.

In a saucepan on the stove, add strawberries and sugar. Bring to a boil over medium heat and stir constantly. Reduce heat and continue to boil as berries break down and soften.

Combine corn starch and water in a separate container. Stir into berries as they boil to thicken. Heat for 1 minute. Remove from heat and stir in butter until completely melted.

Let set to cool for about 15 minutes on the counter and then move to the refrigerator to cool completely.

Place filling into a piping bag fitted with a metal tip. This is important as the tip needs to be on the bag so that it's stiff and can puncture the cupcake. Pipe