Preheat oven to 350 degrees Fahrenheit and line a baking sheet with parchment paper.

In a large bowl, cream together softened butter and granulated sugar until light and fluffy.

Beat in the egg and vanilla extract.

Whisk together flour, cornstarch, baking powder, and salt in a separate bowl and gradually mix into the wet ingredients.

Scoop 1/4 cup portions of dough, roll into balls, and place on the baking sheet, pressing slightly to flatten.

Bake for 12 to 14 minutes until edges are just set but not browned then allow to cool completely.

Whip heavy cream and powdered sugar together until stiff peaks form.

Spread a layer of strawberry jam onto each cooled cookie.

Pipe or spread the whipped cream frosting over the jam layer.

Top with fresh diced strawberries before serving.