Marbled Holiday Sugar Cookies

In the bowl of an electric mixer fitted with the flat beater, beat together the butter and granulated sugar on medium speed until light and fluffy, about 4 minutes. Add the egg yolks one at a time, beating well after each addition. Add the vanilla bean paste and beat until combined, about 30 seconds. Stop the mixer and scrape down the sides of the bowl.
 
In a sifter, combine the flour, baking powder and salt. Sift the flour mixture directly onto the butter mixture and beat on low speed until well mixed, about 1 minute.
 
Remove half of the dough from the mixer bowl and set aside. Add a few drops of red food coloring to the dough remaining in the bowl. Beat on medium speed until the food coloring is combined and the dough is red throughout, about 1 minute.
 
On a lightly floured surface, divide each dough half into 4 equal portions, then roll each portion into a ball. On your work surface, place a red dough ball next to a plain ball and press the sides gently together. Place a second plain ball next to the red ball, on the end farthest from you, then place a second red ball next to the plain one, again on the end farthest from you. Repeat with the remaining dough balls, alternating the colors and pressing the balls together as you go.
 
Using a lightly floured rolling pin, roll out the dough into a rectangle about 11 by 6 1/2 inches (28 by 16.5 cm). Starting with the shorter end of the rectangle farthest from you, fold the dough toward you two times, then roll out into the same-size rectangle. Repeat the folding and rolling four more times. Form the dough into a square-shape log for square cookies, or into a round log for round cookies.
 
Wrap the dough in plastic wrap and refrigerate for at least 2 hours or up to 3 days. Alternatively, freeze for at least 1 hour or up to 2 months.
 
Position 1 rack in the upper third and 1 rack in the lower third of an oven and preheat to 350°F (180°C). Line 2 baking sheets with parchment paper or silicone baking mats.
 
In a small bowl, whisk the egg until blended. Pour the caster sugar onto a large plate or a baking sheet.
 
Remove the dough from the plastic wrap. Brush the dough with the egg wash, then roll in the caster sugar until coated. Using a sharp knife, cut the dough into slices 1/2 inch (12 mm) thick and transfer to the prepared baking sheets, spacing them about 1 inch (2.5 cm) apart.
 
Bake until the cookies are golden on the edges, 10 to 12 minutes, rotating the baking sheets halfway through the baking time. Transfer the cookies to wire racks and let cool completely. The cookies can be stored in an airtight container at room temperature for up to 1 week. Makes 22 to 24 cookies.

Williams Sonoma Test Kitchen