I used this recipe for the Paleo version, but I'll also write the "original" challenge recipe for you to try:
Basic short crust recipe
All Purpose flour- 1 1/4 cup
Confectioner's sugar- 2/3 cup
Pinch of salt
Cold unsalted butter cut into half inch cubes- 10 tbs (1 1/4 stick)
For the filling
Sugar- 3/4 cup
Lemon zest- 1 tbs
Lemon juice-1 cup
Condensed milk- 1/2 cup
(or milk 1/2 cup. Milk or condensed milk is optional it gives the bars a custard flavor and firmer texture)
All purpose flour- 3 tbs
Salt- 1/8 tsp
Yellow food color- 2-3 drops (optional)
Powdered sugar for dusting
To make the crust
Pre heat oven to 325°F and position rack in the center of the oven.Butter a 9 inch square baking pan and set aside (you can also line pan with aluminum foil)
Using a food processor,mix flour,sugar and salt.Add pieces of butter and pulse 8-10 times till the mixture resembles coarse meal. Process until the mixture forms large clumps and holds together when pinched between two fingers (around 15-20 seconds).
Scrape dough into prepared pan and pat it into an even layer.Using a fork prick the dough at one inch intervals.
Bake the crust for 20 minutes till the edges become golden brown. Allow this to cool in the pan on a wire rack.
To make the filling
Pulse sugar and lemon zest in a processor till it is fragrant. Beat eggs and mix the sugar and whisk till it is pale yellow color. Add the condensed milk, lemon juice, flour and salt.
Pour over the baked crust. Bake at 325°F for18-20 minutes or until just set and firm to touch. Cool in room temperature and the place in the refrigerator to chill for 1 hour before cutting into bars.
To finish dust the top of each bar with powdered sugar and serve. Store lemon bars tightly warped in room temperature for two days or in the freezer for one week