Butter Cookies with Orange & Chocolate Ganache

(2 sticks; 230g) unsalted butter, softened to room temperature

confectioners' sugar

and 1/2 Tablespoons orange zest (about 1 orange)

large egg, at room temperature

large egg yolk, at room temperature

and 1/2 teaspoons pure vanilla extract

salt

all-purpose flour, plus more as needed for rolling

optional: coarse sugar for topping