BLUEBERRY JAM SANDWICH COOKIES

Steps:

• For jam:
• Combine blueberries, fruit pectin, and butter in large nonstick skillet. Stir constantly over medium-high heat until mixture boils. Add sugar and return to boil, stirring until sugar dissolves. Boil until mixture is reduced to 1 1/3 cups, about 5 minutes. Remove from heat. Transfer mixture to small glass bowl; cover and chill until jam is cold, at least 6 hours (jam will thicken slightly while chilling). DO AHEAD: Can be made 1 week ahead. Keep chilled.
• For cookies:
• Using electric mixer, beat butter, 1 cup sugar, lemon peel, and salt in large bowl until light and fluffy, about 2 minutes. Add yolks and beat until blended. Add flour in 2 additions, beating just until blended after each addition. Gather dough together; divide in half. Place 1 dough half on sheet of waxed paper or parchment paper. Form dough into 1 1/2-inch-wide square or round log; smooth with dampened fingers. Wrap log in waxed paper. Repeat with second dough half. Chill dough until firm, at least 4 hours. DO AHEAD: Can be made 1 day ahead. Keep chilled.
• Position rack in center of oven and preheat to 400°F. Line 2 large rimmed baking sheets with silicone baking mats, smooth side up, or parchment paper. Cut dough logs into scant 1/4-inch-thick squares or rounds. Arrange on prepared baking sheets, spacing 2 inches apart. Sprinkle half of dough squares or rounds generously with raw sugar (leave remaining plain).
• Bake cookies, 1 sheet at a time, until edges are light golden, 10 to 12 minutes. Transfer cookies to rack and cool completely. DO AHEAD: Can be made 3 days ahead. Store airtight at room temperature.
• Just before serving, spread 1 generous teaspoon jam over bottom (flat side) of each plain (not sugared) cookie. Top each with sugared cookie, sugared side up, and serve.
• Also called turbinado or demerara sugar; available at most supermarkets and at natural foods stores.