Add flour, baking powder, lemon zest and softened butter. Mix well until even crumbly texture.
Slowly add ¼ cup milk and knead until the crumbly mixture starts to come together.
It’ll be very sticky first but will smooth out as you knead it for a few minutes.
Add more milk if the dough is still to hard to knead.
Cover the dough ball with a plastic wrap and let rest in the fridge at least for 1 hour.
In the meantime prepare the filling.
Beat egg yolk with sugar and corn starch. Add milk.
Transfer the mixture to a small pan and place it over medium heat. Stir continuously until creams thickens.
Remove from heat and transfer to a cold bowl. Add a tablespoon of lemon juice or limoncello liqueur.
Once the cookie dough is chilled divide it into 4 parts.
First, roll out 2 pieces of dough on a floured surface to approx ⅓ inch thick sheet.
Using a cookie cutter mark cookie shapes on the first piece of the dough.
Place a dollop of chilled cream in the center of each cookie shape. Use egg wash to brush the edges of each cookie. It'll help seal the cookies better.Egg wash is not necessary if you don't exaggerate flouring the the rolled dough.
Cover cookie sheet with cream dollops with a second piece of the dough. Using same cookie shapes cut out cookies all the way through. Make sure all cookies are sealed well.
Repeat the process with the remaining dough and cookie dough leftovers.
Bake cookies in a preheated to 375F oven for 20 minutes.
Once cookies are completely cool, drizzle them with a lemon glaze (optional).
Lemon Shortbread Cookies with Cream Filling https://www.italianrecipebook.com/lemon-shortbread-cookies-cream-filling/