Mix the dry ingredients (flour, cornstarch and salt) together in a large mixing bowl. Stir together until well combined.

In a separate large mixing bowl, add the softened butter and sugar. Using a hand mixer or stand mixer, cream the butter and sugar together for 1-2 minutes. Add the egg yolk, vanilla extract and lemon juice and beat for another minute.

Pour the dry ingredients on top of the wet ingredients. Stir the batter together until no dry flour remains. It will be dry and crumbly at the beginning, but continue to stir and it will come together. It may take several minutes of hand stirring for the batter to come together.

Cover the bowl with plastic wrap and put it in the refrigerator to chill for at least one hour.

When the dough is done chilling, preheat the oven to 375 degrees F. Line two baking sheets with parchment paper.

Using a small cookie scoop, scoop the chilled dough out of the mixing bowl and roll it into small balls (approximately 1 inch). Put a few tablespoons of granulated sugar on a small plate, and roll the cookies through the sugar. Place the cookie dough balls on the prepared cookie sheet.

Use your thumb to press a small indent into the center of each cookie. Make sure not to press too hard - you do not want to press through to the baking sheet and make a hole. If your cookies crack, just use your fingers to smooth the dough back together.

Use a teaspoon to spoon a small amount of lemon curd into the center of each cookie, so that the cookie is filled to the top of the hole but not overflowing the sides.**

Bake the cookies for approximately 10-12 minutes. The cookies should have barely any color on top, and should be a light golden brown on the bottom. Remove the baked cookies to a cooling rack and allow them to cool completely before eating.