1 large hard-boiled egg, white discarded or reserved for other uses
2 1/4 cups all-purpose flour
6 tablespoons potato starch
15 tablespoons (1 stick plus 7 tablespoons) unsalted butter, at room temperature
1/3 cup olive oil (a fruity oil is best)
1 cup confectioners’ sugar, sifted
2 1/2 ounces (about 1/2 cup) pitted black olives, preferably oil-cured, chopped
Grate the hard-boiled egg onto a piece of wax paper. Put the flour and potato starch in a strainer set over a large bowl and sift into the bowl; whisk to thoroughly blend.
Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter on medium speed until it’s soft and creamy. Beat in the olive oil, followed by the grated yolk. Blend in the confectioners’ sugar, reduce the speed to low and add the dry ingredients. Mix until the dough just comes together—there’s no reason to beat this dough, and you shouldn’t—then stir in the chopped olives. You’ll have a soft, pliable dough. (If you prefer, you can make the dough by hand, using a rubber spatula to blend the butter, oil, yolk and sugar and to fold in the dry ingredients and olives.)
Turn the dough out onto a work surface, divide it into thirds, and shape each piece into a log about 1 1/2 inches in diameter. Wrap the dough in plastic and chill for several hours or, better yet, overnight. If you’re in a hurry, you can freeze the logs for an hour or so.
When you’re ready to bake the sablés, center a rack in the oven and preheat the oven to 325°F. Line a baking sheet with a silicone baking mat or parchment paper.
Working with 1 log at a time, slice the cookies 1/4 inch thick and arrange them on the baking sheet—you want to bake these one sheet at a time.
Bake the sablés for 15 to 18 minutes, rotating the baking sheet at the midway mark, or until the cookies are firm, but not colored. They may turn golden around the edges, but you don’t want them to brown. Transfer the cookies to a rack to cool, and repeat with the remaining logs of dough, making sure to use a cool baking sheet each time.
Makes about 60 cookies.