In a large bowl, sift together flour, baking powder, and salt. Beat butter until fluffy, about three minutes. Add sugar and beat until light and fluffy, about two more minutes. Add eggs one at a time, beating after each addition, then beat in vanilla. Add flour and beat on low until combined.

Divide dough in half. Remove half from mixer. Melt chocolate in double boiler or microwave. Add melted chocolate to dough in mixer and beat just until combined.

Divide vanilla dough into three pieces. Place each piece on a large piece of plastic wrap. Pat it into a 1/2-inch- (1.3-centimeter-) thick square and then wrap it in the plastic. Repeat for the chocolate dough. Refrigerate all six pieces until firm (about one hour).

Unwrap one piece of the vanilla dough, leaving it on the plastic. Cover it with a second piece of plastic wrap, then roll it into a rectangle measuring about 9 x 9.5 inches (23 x 24 centimeters). If the dough gets too soft or difficult to roll, slide it onto a baking sheet and refrigerate it until it’s firm again (5–10 minutes). Unwrap a piece of the chocolate dough, cover it with another piece of plastic and roll it into a 9- x 9-inch (23- x 23-centimeter) square.

Remove the top piece of plastic wrap from both rolled doughs and invert the chocolate dough on top of the vanilla dough, lining up three sides and leaving about a 1/2 inch (1.3 cm) of vanilla dough exposed on one side. Starting at the opposite side, lift the vanilla dough, using the plastic wrap underneath it, and snugly roll up both layers into a log, peeling away the plastic wrap as you go. Wrap the log in the plastic and refrigerate for at leasttwo hours. Repeat the process with the remaining pieces of dough.

Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius). Line two baking sheets with Gefen Parchment Paper.

Leave the cookies on the baking sheet for one minute, then transfer them to a wire rack to cool. Slice and bake the other logs like the first.

Unwrap one of the logs and slice it into 1/4-inch- (.6-centimeter-) thick rounds. Arrange the slices on the parchment about 1 and 1/2 inches (3.8 centimeters) apart and bake them for 11–12 minutes. They should be a little soft to the touch – the cookies will have a better texture if you don’t let their edges brown.

Divide vanilla dough in half. Place each piece on a large piece of plastic wrap. Pat into a 1/2-inch- (1.3-centimeter-) thick square and then wrap it in the plastic wrap. Repeat process with the chocolate dough. Refrigerate all four pieces until firm (about one hour).

Unwrap one piece of the chilled dough, leaving it on the plastic. Cover it with a second piece of plastic wrap, then roll it into a rectangle measuring about seven inches (18 cm) wide and eight inches (20 cm) long. If the dough is too soft and difficult to roll, slide it onto a baking sheet and refrigerate it until it is firm again (5–10 minutes). Unwrap a piece of the chocolate dough, cover it with another piece of plastic, and roll it into a rectangle seven inches wide and eight inches long. Remove the top piece of plastic wrap from both rolled doughs and invert the chocolate on top of the vanilla dough.

For a striped design, cut dough in half lengthwise into two strips. Brush one strip with egg white and put the other on top. Continue cutting and stacking the strips until there are eight layers of chocolate and vanilla dough. Trim the end, wrap the log in plastic, and neaten its shape with the flat of your hand. Chill for about two hours.

Slice into squares 1/4 inch (.6 centimeter) thick and bake as directed above for spiral cookies.

For a checkerboard design, cut dough in half lengthwise into two strips. Brush one strip with egg white and put the other on top. Continue cutting and stacking the strips until there are eight layers of chocolate and vanilla dough. Using a small, sharp knife, slice the layered dough lengthwise into four or five strips. If the dough becomes warm, chill for 10 minutes before continuing.

Brush one strip with egg white. Place the next strip on top, so a vanilla strip appears on top of a chocolate one, giving a checkerboard effect. Repeat until all the strips are used. Trim the ends, wrap the log in plastic, and neaten its shape with the flat of your hand. Chill for about one hour.

Slice into squares 1/4-inch thick and bake as for spiral cookies.

Roll all scraps of dough into logs. Refrigerate in plastic wrap for two hours. Unwrap log and cut into 1/4-inch rounds. Bake as for spiral cookies.