1/2 cup unsalted butter, softened
1 cup packed dark brown sugar
1 large egg
1 teaspoon vanilla extract
1 cup all-purpose flour
3/4 cup whole wheat flour
1/4 teaspoon table salt
1/2 teaspoon baking soda
Preheat oven to 400°F.
In a large bowl, using an electric mixer, cream butter, sugar, egg and vanilla extract. Add remaining ingredients and beat thoroughly.
Scrape dough out of bowl and onto a large piece of plastic wrap. Roll dough into a log, about 2 1/2-inches wide; completely wrap dough in plastic and refrigerate until firm, at least 20 minutes or up to overnight.
Cut dough into 1/8-inch-thick slices and arrange on several large ungreased cookies sheets about 1- to 1 1/2-inches apart. Bake until lightly browned around edges, about 8 to 10 minutes.
Allow cookies to cool on cookie sheet for about 1 minute and then remove to a wire rack to cool completely. Yields 1 cookie per serving.
Notes: Add some spice to these cookies by sprinkling a pinch of cinnamon and nutmeg into the batter.