Chocolate Sandwich Cookies with Butter Cream Filling

Ingredients:
For the cookies:
1 1/4 sticks butter, softened
3/4 cup sifted powdered sugar
3 squares semi-sweet chocolate (3 oz), melted and cooled
2 tbsp. cornstarch, sifted
1 cup sifted all-purpose flour
1 cup sifted self-rising flour

For the butter-cream filling:
1 stick butter, softened
1/2 tsp vanilla extract
2 cups sifted powdered sugar

Directions:
Preheat the oven to 350 degrees.
To make the cookies:
Beat the butter and sugar together until lightened in color and texture. Beat in the melted chocolate, then stir the cornstarch in. In a separate bowl, sift together the flours, and add to the chocolate mixture in three parts, mixing after each addition. Spoon the batter into a ziploc bag, and cut off one corner. Squeeze the batter in spirals onto a greased cookie sheet. Bake for 12-15 minutes. Cool the cookies completely.
To make the butter cream:
Beat together the butter and the vanilla extract. Beat in the powdered sugar. Spread the butter cream on top of the cookies and make sandwiches.

Source:
Atkinson, Catherine, Juliet Barker, Carol Tennant, et al. Simple Chocolate: Step-by-Step. San Diego, CA.: Thunder Bay Press, 2001. Page 54.