Preheat oven to 375 degrees F.

Melt the chocolate in a small heavy saucepan over low heat and set aside.

Sift together the flour, baking powder, and cinnamon, and set aside.

In electric mixer, cream together butter and 2/3 cup sugar.

Add the melted chocolate and the egg and beat well.

Dissolve the Colman's Mustard in the heavy cream and add to the chocolate mixture.

On the lowest speed, fold in the sifted flour mixture.

Divide the dough in half. Place each half on a sheet of waxed paper and form into a 10-inch strip.

Using the waxed paper to help, lift and roll each strip into a 10-inch cylinder. Wrap each cylinder in the waxed paper, twisting the ends to secure.

Chill for two hours.

Slice in ¾ inch rounds.

Bake for 15-20 minutes. Cool and enjoy!