Preheat oven to 350℉ and line a baking sheet with parchment.
Combine sugar and margarine, mixing well. Add vanilla extract and the egg, whisking together. Mix in lemon juice. Set aside.
In a medium sized bowl, mix flour, cornstarch, baking powder and salt. Add to wet ingredients a small amount at a time, mixing in between until combined.
Take a large tbsp sized scoop of batter, roll into a ball and place on cookie sheet. Flatten slightly and make a thumbprint sized indent in the center, adding a small scoop of jam.
Bake for 10-12 minutes, be sure not to overcook. Let cool and enjoy!