Step 1

Preheat oven to 350°F (177°C). Line 2 large baking sheets with parchment paper (or silicone baking mats). Set aside.

Step 2

In a large bowl of a stand mixer fitted with the paddle attachment, or a large mixing bowl using a handheld mixer, beat butter and sugar for 1-2 minutes until pale and fluffy. Use a spatula to scrape any ingredients from the side of the bowl.

Step 3

Add egg yolk, vanilla extract, and salt and mix well until fully combined.Add sifted flour and mix slowly. The mixture will be a little crumbly at first but the cookie dough will come together as you continue mixing it.

Step 4

Using a cookie scoop or one tablespoon, scoop cookie dough, roll into balls and roll each one in granulated sugar. Place on the prepared baking sheets, leaving a 2-inch space between each one.

Step 5

Use your thumb or the back of a rounded measuring spoon to press an indentation into the center of each ball of cookie dough. Repeat for the remaining cookie dough.

Step 6

Bake for 11-13 minutes or until the cookies are set and the bottoms are lightly browned. Remove from the oven and repeat the indentation step if previous indentations are gone or decreased in size. Do this carefully and using very light pressure as the cookies will be soft when out of the oven.

Step 7

Cool on the baking sheet for 10 minutes, then transfer the cookies to a wire rack to cool completely before filling.

Step 8

In a medium-sized bowl, beat together the egg, egg yolk, sugar, lemon zest and juice. Transfer mixture into a small saucepan and stir over low-medium heat until the sugar has dissolved.

Step 9

Gradually add the cold cubed butter, stirring until each addition is melted. Make sure not to let the curd come to a boil or bubble at any point.

Step 10

Continue cooking the mixture, stirring frequently using a wooden spoon for a few minutes. Lemon curd is ready once it is thickened and coats the back of a spoon or when it leaves a thick coating on the side of a tilted saucepan.

Step 11

Pour the cooked lemon curd through a fine-mesh strainer into a clean bowl to remove any lemon zest and other bits.

Step 12

Let the lemon curd cool for 10-15 minutes. It will continue to thicken as it cools down.

Step 13

To fill the cookies, carefully spoon 1-1½ teaspoonfuls of cooled lemon curd into the indentation of each thumbprint cookie. Dust with powdered sugar and store in a refrigerator until ready to serve.