In another bowl, cream the butter, sugar, lemon zest and juice with an electric mixer. Add the egg and egg yolk. Mix to combine. With the machine on low speed, or with a wooden spoon, stir in the dry ingredients until smooth.
Form the dough into 2 discs. Cover in plastic wrap and refrigerate for 1 hour.
With the rack in the middle position, preheat the oven to 375°F (190°C). Line two baking sheets with silicone mats or parchment paper.
On a floured work surface, roll out one disc of dough at a time until ¼ inch (5 mm) thick. Cut out cookies using cookie cutters of your choice, reusing any dough scraps. Arrange the cookies on the baking sheets.
Bake one sheet at a time for 12 to 15 minutes or until the bottoms and sides of the cookies are lightly golden. Let cool completely on the baking sheet, about 1 hour.
Icing and Decorations
In a bowl, whisk together all of the ingredients, except for the sprinkles, until smooth.
Dip the tops of the cookies in the icing or, using a pastry bag, decorate the tops of the cookies as you like. Garnish with sprinkles, if desired. Let set on a wire rack for 2 hours. The cookies will keep for 1 week in an airtight container at room temperature.