HOW TO KNEAD COOKIE DOUGH RECIPES

Steps:

• Whisk flour, salt, and baking powder in a medium bowl until combined. Lightly coat a 9-by-5-inch loaf pan with cooking spray. Line with plastic wrap, tucking corners and leaving a generous overhang on all sides.
• Beat confectioners' sugar and 1 cup butter in a large bowl with an electric mixer on medium-high speed until light and fluffy, 3 to 4 minutes. Add egg and vanilla; continue beating on medium-high, scraping down sides of bowl as needed, until incorporated, about 1 minute. Reduce speed to low and add flour mixture, beating until dough comes together, about 1 minute.
• Transfer dough to a lightly floured work surface. Divide dough into thirds; return two-thirds of dough to bowl. Add remaining ¼ cup butter to dough in bowl and beat on medium speed until fully combined, about 1 minute. Remove dough from bowl; set aside. Add remaining third of dough and food coloring gel to bowl; beat on medium speed until fully combined, about 1 minute. Transfer dough to a lightly floured work surface.
• Cut larger dough portion in half; shape each half into a 5-inch square. Cut blue dough in half; shape each half into a 5-inch square. Stack, alternating colors, and gently press to form 1 dough piece. Cut dough in half crosswise. Gently knead and roll each dough piece until marbled. Stack both pieces together; gently knead to form 1 dough. Press dough into prepared loaf pan; fold over hanging plastic wrap over top to seal. Chill until firm, at least 4 hours and up to 3 days. (Or freeze for up to 3 months.)
• Preheat oven to 325°F with racks in upper and lower thirds. Line 2 large, rimmed baking sheets with parchment paper. Whisk egg white and 1 teaspoon water in a small bowl. Spread sanding sugar on a plate. Remove dough from loaf pan; unwrap. Trim slanted sides and ends of dough, if desired, for near-perfect rectangular cookies. Cut dough in half lengthwise. Lightly brush long sides of each piece of dough with egg white mixture. Press long sides of dough firmly into sanding sugar to cover completely.
• Cut dough crosswise into ¼-inch-thick slices; transfer to prepared baking sheets, placing cookies 1 inch apart. Freeze until firm, about 10 minutes. Bake cookies until golden underneath and along edges, 14 to 16 minutes. Let cookies cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely. Store cookies at room temperature in an airtight container for up to 1 week, or freeze for up to 1 month.