While this classic style of French butter cookie would normally be made with salted butter, I prefer to use unsalted butter and then add the salt myself (different brands of butter can have different amounts of salt). These short-bread cookies are barely sweet—really, it’s all about the butter!
Makes 2 1/2 dozen cookies.
Preheat the oven to 300°F (150°C) and line the cups of mini-muffin pans with paper liners (petits fours size).
Using electric beaters or a stand mixer fitted with the paddle attachment, beat the butter and icing sugar on medium speed until smooth and light. Beat in 2 of the egg yolks. Add the flour, cornstarch and 1 tsp salt and mix on low speed until the dough comes together.
Use a small ice cream scoop or 2 teaspoons to drop the dough into the lined muffin pans. Use the bottom of a glass dipped in flour to gently press the dough flat. Whisk the remaining egg yolk with 1 Tbsp (15 mL) of cold water and brush the tops of the cookies. If you wish, sprinkle the cookies with a little flaked sea salt.
Bake for 35 to 45 minutes, until the tops are a rich golden brown. Let cool in the pans on a wire rack.