Start by making your jam: Stir together all of your ingredients in a pot and place over medium-high heat, make sure to stir regularly. Once the mixture starts to boil, use your thermometer to check until it reaches 220F, stirring continuously. Immediately remove from the heat and pour into a heat-safe bowl, cooling to room temperature. If making in advance, store in an airtight container in the fridge

Line a baking sheet or large plate (whatever will fit into your fridge) with parchment paper

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy, about 2 minutes

Add the egg, lemon juice, and lemon zest, and beat until fully combined

In a separate bowl, whisk together the flour, cornstarch, salt, and baking powder

Gradually add the dry ingredients to the wet mixture, mixing until just incorporated

Scoop out tablespoon-sized portions of dough. This is a sticky dough so lightly dampen your hands before rolling them into balls

Place the dough balls on the prepared baking sheet or a large plate

Dampen your thumb and press into the center of each dough ball to form a slight indent, about two-thirds of the way in. You don’t want to press through to the bottom, just enough to make a well for your jam

Place in the fridge for 20 minutes, halfway through preheat your oven to 350F

Remove the cookies from the fridge and place half on a parchment-lined baking sheet, placing the rest in the fridge until ready to bake. Using a spoon, fill the center of each cookie with your jam

Place your cookies on a parchment-lined baking sheet about 2 inches apart and bake for about 8 minutes

Let the cookies cool on the baking sheet for two minutes before transferring them to a wire rack to cool completely

Enjoy!