Add the butter, powdered sugar and lemon zest to a large mixing bowl. Beat with a hand mixer or a stand mixer fitted with the paddle attachment just until the ingredients are combined.
Add in the egg yolk and vanilla and beat until just combined.
Add in the flour, almond flour, and salt. Beat just until a cohesive dough forms.
Form the dough into a square and tightly wrap it in plastic wrap. Transfer the dough into the fridge to chill for 30 minutes.
In the meantime, make the lemon curd.
Lemon Curd
Add the egg yolks, sugar, lemon juice, lemon zest, and salt to a small saucepan.
Whisk the mixture vigorously over medium heat until it thickens, and reaches at least 170°F / 76°C. (This took me about 3-5 minutes)
Remove the curd from the heat and pass it through a fine mesh sieve to remove the zest and any accidental cooked egg.
Immediately whisk in the cubed butter until it melts and the mixture is completely smooth.
Transfer the lemon curd to the fridge to chill until you're ready to assemble the cookies.
Linzer Cookies
Once the dough has chilled, remove it from the fridge and place it on a clean, floured work surface.
Roll the dough out until it is ¼" thick. Use a 2" linzer cookie cutter to cut out the cookies.
Gather the scraps of dough and re-roll to ¼" thick, and cut out more cookies. Repeat 1 more time.
At this point you should have 28 cookies. To half of the cookies, cut out a little design in the middle with a smaller cookie cutter (I used a bunny!).
Carefully transfer all of the cookies to a cookie sheet that is lined with parchment paper. Place the sheet in the freezer to chill for 10 minutes, while you preheat the oven to 350°F / 175°C
Once the cookies have chilled and the oven is preheated, bake them for 10-12 minutes, or until the cookies no longer look "wet"
Allow the cookies to fully come to room temperature before filling them.
Assembly
The cookies that have no design stamped out are your bottom cookies, and the cookies with the design are your top cookies.
Arrange all of the top cookies on a tray and give them a dusting of powdered sugar.
Flip all of the bottom cookies upside down. Dollop ~¾ tsp. of lemon curd in the center of the cookie, then top it off with the top cookie.