Lemon Bar Sheet Cake with Lemon Curd and Shortbread Crust

A Luscious Lemon Bar Sheet Cake with a zesty homemade Lemon Curd, Shortbread Crust on bottom, and a tasty lemon buttercream.

Servings 15 Large Square Slices

3 cups shortbread cookie powder (11-15oz pulsed into a powder and then 3 cups measured out)
5 Tablespoons unsalted butter melted
1 pinch salt
zest of 1 fresh lemon (about 1 Tablespoon)
3/4 cup buttermilk room temperature
2/3 cup sour cream room temperature
3 whole eggs room temperature
1 egg white room temperature
1 Tablespoon vanilla
1 teaspoon lemon extract optional
1/3 cup vegetable oil
1/4 cup all purpose flour
1 15.25oz Box Lemon Cake Mix (Duncan Hines is preferred)

1 1/2 cup unsalted butter left out of fridge for 15 minutes or so1 teaspoon lemon extract
1 Tablespoon vanilla
1 pinch salt
1 teaspoon lemon extract I used PK Lemon Drop Elixir
1/4 cup heavy cream
6-7 cups powdered sugar
1 drop yellow gel coloring

Prep a 9x13 sheet pan with a swipe of shortening and dust of flour to prevent sticking. Preheat the oven to 325 degrees. Set aside.
For the crust, pulse about 15oz cookies into a powder, then measure out 3 cups into a bowl. Add in melted butter and salt, and stir until all the butter has been incorporated into the cookie powder for a fine crust.
Press the crust into the prepared 9x13 sheet pan and flatten evenly with the bottom of a glass or flat measuring cup. Set aside. There's no pre-bake needed.
For the cake, hand-whisk together all the ingredients except for the cake mix. Sift in the cake mix, and gently stir until incorporated. Pour directly onto the shortbread crust and bake for 28-30 minutes. Let cool in the pan completely before adding on the decoration. Make the curd while the cake is baking.

In a medium saucepan over medium heat, stir together the yolks, sugar, juice, and zest. Whisk together as the mixture begins to thicken, about 6 minutes. Curd should be thickening, and will be thick enough to coat the back of a spoon.
Remove curd from the heat, and stir in the butter cubes. Stir to combine until the butter has melted. Strain the curd, and discard the zest.
Cover the curd with a light layer of plastic wrap placed directly on top of the surface of the curd, and then store in the fridge until cooled. You’ll want it cold before adding it to your cake.

In a stand mixer fitted with a paddle attachment, whip up butter for about a minute until light and fluffy. Scrape down the sides of the bowl.
Add in salt, lemon extract, vanilla, almond extract, and heavy cream until combined. Slowly add in powdered sugar, about a half cup at a time. Add in the lemon yellow gel coloring. Add in more heavy cream if needed for a thinner consistency. Whip on high for a minute to create a soft buttercream, then back down to low for 30 seconds to beat out any air bubbles.

Add 1 1/2 cups of buttercream to a piping bag fitted with a 1M piping tip, set aside.
Spread on the rest of the buttercream in a flat layer on top of the 9x13 cake. Use an offset icing spatula or cake scraper to make a flat, even layer.
Pipe on long lines of buttercream shells in a straight pattern.
Fill between the pattern with the lemon curd (you'll have a little extra, depending on how much curd you use).
Sprinkle on the crumbled shortbread on top. Enjoy!