A dip of creamy white chocolate balances the bite of grapefruit in Yossy's cutout cookies.
Combine butter, confectioners sugar, and grapefruit zest in a large bowl. Beat on medium until smooth and creamy, 2 minutes. Add egg yolk and grapefruit juice; beat until just combined.
Sift flour, cornstarch, and salt into butter mixture. Mix on low until almost combined. Use a rubber spatula to stir until combined. Wrap dough in plastic wrap and chill until firm, about 1 hour.
Preheat oven to 350°F. Line two cookie sheets with parchment paper. Dust work surface with flour. Working with half of the dough at a time, roll to approximately 1/4-inch thick; cut out using desired 2-inch cookie cutter(s). Reroll scraps. Place on prepared sheets.
Bake cookies until edges are golden, 9 to 10 minutes. Let cool completely on baking sheets.
Place chocolate in a small glass bowl and melt in microwave in 20-second bursts. Dip cookies in melted chocolate or drizzle over cookies. If desired, sprinkle with decorating sugar or sea salt flakes. Let cookies stand until chocolate is firm, about 30 minutes. Makes 32 cookies.
To store: Layer cookies in an airtight container. Store at room temperature up to 24 hours, or chilled up to 3 days; freeze plain cookies up to 1 month.