Instructions:

Dough:
In a large bowl, combine flour, sugar, salt, and lemon zest.
Add the cold butter and work the dough with your hands until it resembles coarse crumbs.
Add the whole egg and egg yolk, and knead briefly until a dough forms.
Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.
Streusel:
In a separate bowl, combine flour, sugar, and cinnamon.
Add the cold butter and rub it in with your fingers until you have a crumbly mixture.
Assembly: