Line two baking sheets with parchment; set aside. Preheat oven to 350 degrees F. In a small bowl whisk together the flour, the 1/2 tsp. allspice, baking powder, and salt. Set aside.
In a large mixing bowl beat butter with electric mixer on medium speed for 30 seconds. Add the sugar and beat for 2 minutes or until well blended. Reduce mixer speed to low. Add the egg yolk and beat for 1 minute. Beat in vanilla.
With mixer at lowest speed, steadily beat in flour mixture, beating only a few seconds after flour is well incorporated into the soft dough.
Shape rounded teaspoons of dough in balls between palms of hands. Place balls about 2 inches apart on prepared baking sheets. With the handle end of a wooden spoon or a thumb, make indentations in the center of each ball.
Bake for 9 to 10 minutes or until cookies are dry and firm to the touch. For even baking, rotate baking sheets top to bottom and front to back after 4 minutes of baking time. Cool cookies on baking sheets on wire racks while preparing filling.
For filling, in a small microwave-safe bowl melt chocolate pieces on high (100% power) for 45 to 60 seconds, stirring until melted. Use a small spoon to fill each indentation with chocolate. Sprinkle with ground allspice, if desired. Refrigerate cookies for 15 minutes to set chocolate. Serve cookies at room temperature.