Make the crust:
Preheat your oven to 350 degrees F. Line an 8-inch square baking pan with a foil sling.
In a food processor, place the flour, powdered sugar, and salt process together for a few seconds before adding in the cubed cold butter.
Cut the butter into the flour mixture with about 6-8 pulses or until it looks like small peas. Press the mixture into the baking dish with the flat bottom of a glass or small dish.
Bake for 20 minutes or until lightly browning. When you remove the crust from the oven, increase the oven temperature to 375 degrees F.
Make the streusel topping:
While the shortbread bakes, combine the flour and sugar in a food processor by pulsing a few times.
Add the cubed butter and pulse 6 or 7 times until the butter is the size of small peas and is coated with the flour/sugar mixture. Place the streusel in the fridge until ready to use it.
Make the filling:
With an electric mixer beat the cream cheese until it's fluffy.
Add the lemon zest, sugar, lemon juice, and egg, and beat until combined.
Pour the filling over the hot shortbread crust and spread it evenly.
Top with the streusel and spread out evenly. Bake for 20-25 minutes or until the filling registers 160 degrees on a thermometer.
Place the finished bars in the fridge to chill for about 2 hours before slicing and serving.