Step 1

Whisk together the flour and cornstarch, and set aside.

Step 2

Beat together the butter and the sugar until just combined. Beat in the egg yolk until smooth and light.

Step 3

Stir in the flour mixture, switching to your hands when you can no longer work the dough with a wooden spoon. Careful not to overwork the mixture; you want to stop the moment no more streaks of flour remain. Chill the dough for 30 minutes.

Step 4

Pre-heat the oven to 350F and line two large baking sheets with baking parchment.

Step 5

Using a cookie scoop or a measuring spoon, roll the dough into balls that fit perfectly into a round 1 tbsp measure. Space them out on the baking sheet at least 1 1/2 inches apart, and using a 1/2 tsp spoon, make deep indentations in the top of each ball to hold the jam.

Step 6

Fill each cookie indentation with heaped 1/4 tsp spoonfuls of jam.

Step 7

Bake for 20-25 minutes until the cookies are just starting to turn golden. Cool on the baking sheets before transferring to an air tight container.