Add sugar and citrus zest to a stand mixer bowl, use a hand whisk or your fingers to rub the mixture together to release the citrus oil. Add softened butter and beat with the paddle attachment until fluffy. Add egg yolk and vanilla extract, and beat until combined.

In a small mixing bowl, whisk together all-purpose flour, almond flour, salt, and cinnamon. Add flour mixture to the butter mixture and mix until just combined.

Scrape the dough out of the bowl onto a piece of plastic wrap, pat into a flat disc or rectangle, wrap well and refrigerate for at least 1 hour until firm.

Preheat the oven to 350°F.

Bake for 10 - 12 minutes until the edges are just beginning to brown. Let them cool on the baking sheet for 5 - 10 minutes before moving them to a wire rack to cool completely.