Create a double boiler by bringing 1-2 inches of water to a simmer in a small saucepan. Place a heatproof glass bowl on top and make sure it's not touching the water.
Add the egg yolks, sugar, lemon juice, lemon zest, and salt into the top of your double boiler.
Whisk until completely combined and then continue to whisk as the lemon curd cooks (constant whisking prevents the lemon curd from curdling). Continue to whisk the lemon curd for 10-15 minutes or until it become thick and paler in color (it should be similar to the consistency of hollandaise sauce). If the lemon curd isn’t thickening, turn up the heat and constantly whisk.
Once thickened, remove the glass bowl from the heat. Cut the butter into chunks and whisk into the lemon curd until fully combined and smooth.
If desired, pass through fine mesh strainer and then pour into a glass jar. Let cool to room temperature and then cover and store in the fridge for up to a week.
Thumbprint Cookies
Using a stand mixer fitted with the paddle attachment, cream together the softened butter and powdered sugar until light and fluffy. Then add in the egg and vanilla extract and mix until fully incorporated.
Fold in the flour, almond flour, and salt until just combined. The dough should come together and still be slightly sticky.
Line a baking sheet with parchment paper.
Scoop about 3 tablespoons of dough and roll into balls. I use a size 20 cookie scoop.
Lightly flour the back of a 1 tablespoon measuring spoon and press into your dough balls to flatten them slightly and create a deep divot in the top.
Place the baking sheet in the freezer for a minimum of 20 minutes (or the fridge for 40 minutes) before baking to allow the cookies to firm back up. This ensures the cookies won't spread during baking.
While the cookies are chilling, preheat your oven to 350°F.
Bake for 9-11 minutes or until the edges are set and just starting to brown. Bake on the middle rack of the oven and rotate the pan halfway through baking for the best results.
Let the cookies cool on the baking sheet for 2-3 minutes, then remove and let them finish cooling on a cooling rack.
Lightly dust the cookies with powdered sugar (optional).
Then fill with about 2 teaspoons of homemade lemon curd and enjoy!!
Store in the fridge for up to 5 days.
Notes
I've tested these lemon curd cookies a few different ways to achieve different textures.The method listed in this recipe has you bake the cookies first and then fill with the lemon curd afterwards. This yields the prettiest cookie with bright yellow lemon curd that is still super tangy.However, the lemon curd isn't quite as thick and still very sticky. So the cookies don't stack very well and are harder to store or transport.Dusting the cookies with powdered sugar BEFORE filling with lemon curd, makes for a prettier, cleaner cookie. However, the lemon curd doesn't stick to the powdered sugar well so the cookies are a little less stable if you're going to be transporting them.So alternatively you can fill the cookies with curd and then lightly dust the edges with powdered sugar. Up to you!The other method I tested was to bake the cookies with the lemon curd already in the middle. This led to a more mellow tasting lemon curd that was much paler in the color. However, baking the lemon curd with the cookies led to a thicker lemon curd with a skin forming on top which made the cookies much easier to store and transport. They also didn't look as pretty.A third option that works well is baking the cookies for about 8 minutes, then removing them, adding in the lemon curd, and continuing to bake for another 2-ish minutes. This helps the lemon curd set while maintaining its bright yellow color. The only downside is you still don't get clean powdered sugar lines.Every version taste great, so I recommending baking one of each and deciding which you like better!