Combine chocolate and milk in top of double boiler. Cook over boiling water until blended and thickened, stirring constantly. Add 1/2 cup brown sugar and continue cooking 5 minutes. Cool.
Sift flour once, measure, add baking powder and salt and sift again. Cream butter thoroughly, add 1 cup brown sugar gradually and cream together until light and fluffy. Add egg and beat well. Stir in chocolate mixture, milk and vanilla. Add flour all at once and mix well. Chill overnight or until firm enough to roll. Divide dough in six equal parts. Roll each portion of dough 1/8 inch thick on slightly floured board, into rectangle sheet 4 1/2x12 inches. Spread 1/3 cup filling in 1 1/2-inch strip down center of dough. Fold dough over filling, brush edges with milk, and seal. Place seam-side down on ungreased baking sheet and brush with milk. Bake in moderate ocen (350 degrees F.) 20 minutes, or until done. Let cool in pan 5 minutes, then cut diagonally in bars 1 inch wide. Makes 5 1/2 dozen bars.