4 ounces unsweetened chocolate, chopped
1 cup butter, softened
2 cups white sugar
2 Tbsp vanilla
4 cups flour
1 tsp baking soda
1 tsp table salt
Melt chocolate in a double boiler or in the microwave, then let cool to room temperature.
In large bowl, beat butter with sugar until light and fluffy. Beat in 2 of the eggs, 1 at a time, then beat in vanilla. In separate bowl, whisk together flour, baking soda and salt; stir into butter mixture in 3 additions, using hands if too stiff to stir.
Remove half of the dough for vanilla dough. Stir chocolate into remaining dough in bowl, using hands to blend thoroughly.
Divide vanilla dough in half, and shape into flat squares. Place dough, one square at a time, between 2 sheets of waxed paper; roll into two 7-inch squares. Chill for about 30 minutes or until firm. Using ruler and sharp knife, cut each square into nine 3/4-inch wide strips. Repeat with chocolate dough.
Place about 1-foot long piece of plastic wrap on work surface. Alternating vanilla and chocolate strips, place 3 strips of dough directly on plastic wrap, close but not touching. Whisk remaining egg; brush over sides and tops of strips. Gently press strips together to adhere. Repeat, forming second and third layers and alternating placement of strips, to create checkerboard effect. Gather plastic wrap up to cover log; refrigerate for at least 30 minutes or for up to 4 days. Repeat for second log, reversing colour pattern.
Heat oven to 350.
With sharp knife, trim ends and cut each log into 1/4-inch thick slices. Arrange 1-inch apart on parchment paper-lined rimless baking sheet. Bake in centre of oven, rotating trays halfway through, for about 12 minutes or until set and very light golden. Let cool on pan on rack for 3 minutes. Transfer to racks to let cool completely.