Sift the flour and cornstarch into a large bowl.
Grate the hard-boiled yolk (or mash very well) and reserve.
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, then stir in the chopped olives. You'll have a very soft, very pliable dough.
Turn the dough onto a work surface and divide it into thirds. Shape each piece into a log about 1 ½ inches in diameter or flatten it into discs like pie dough. Wrap the dough pieces in plastic and chill for at least 2 hours. If you're in a hurry, you can freeze the logs for about an hour.
When you're ready to bake the sables, center a rack in the oven and preheat the oven to 325°F (160°C).
Line a cookie sheet with parchment paper.
Working with one log or disc at a time, slice the cookies about ¼ inch thick and arrange them on the prepared baking sheet. If they break, press them slightly together with your fingers. You can also roll and cut them with a round cookie cutter and transfer them to the prepared pan with a spatula.
Bake the sables for about 15 minutes, one sheet at a time, rotating it at half-time. When done, they may turn golden around the edges, but you don't want them to brown. The cookies are firm but not colored.
Transfer the cookies to a wire rack to cool. Repeat with the remaining dough, using a cool baking sheet each time.
Store in airtight tins.