Heat oven to 325 and have a rack ready in the middle of the oven. Butter or spray an 8-inch square baking pan. You can also line with parchment after buttering for easy removal of the whole thing.

Combine all the ingredients except the flour and mix well. Add your flour, and stir, with a wooden spoon or spatula until all combined. It will be thick and seem like a cookie dough. Press the dough into your greased baking dish and push down until basically even, poke your holes with a fork in the crust. Do this part gently, you don't have to poke all the way through the bottom.  Bake for 25 minutes or until lightly brown. Remove from the oven.

For the filling, add your sugar and lemon zest to a bowl and with your flingers smush together - this sounds odd, but it will release the oils in the zest and flavor the sugar nicely with extra lemon. Whisk the four into the sugar. Add the remaining ingredients and whisk until totally combined. Pour this onto the warm shortbread and continue to bake about 45-50 minutes to fully set the filling.