In a large bowl whisk together the flour, baking soda and salt. Set aside.
In a separate bowl beat the butter and sugar until light, using a hand mixer.
Add the egg, lemon zest and vanilla extract and beat until just combined.
Fold in the flour mixture and beat until you have a smooth dough.
Divide the dough in half. Wrap each piece in plastic wrap and freeze for 10 minutes or until firm.
Preheat oven to 350 F. Line 2 baking sheets with parchment paper.
On a lightly floured surface, roll out the dough to ¼-in. thickness. Cut out cookies using a coffee mug cookie cutter and transfer to the prepared baking sheet.
Bake for 10 minutes or until they begin to brown around the edges.
Remove from the oven and allow to cool for 5 minutes, then transfer to a wire rack to finish cooling.
Decorate the cookies with royal icing if desired. Let the icing dry completely before storing the cookies.