Lightly butter two 9-inch round cake pans. Set aside.
In the bowl of an electric mixer, cream the butter and sugar on high speed until light and fluffy, 3 to 4 minutes.
Sift the flour, cinnamon, allspice, and salt into a medium mixing bowl.
With the mixer on low speed, carefully add the flour mixture to the creamed butter mixture in 3 batches, waiting for each addition to be incorporated before adding the next. Continue mixing only until the flour is just incorporatedóif you overmix, the shortbread will be tough.
Divide the dough evenly between the two prepared cake pans and, using your fingers, press the dough so that it forms a smooth layer covering the entire bottom surface of the cake pan. Using the tines of a fork, prick the pastry all over at regular intervals, pressing all the way through to the bottom. Press the tines of the fork all around the outer edges of the pan. Using a sharp paring knife, score the pastry rounds, cutting them only part of the way through to form 8 even wedges in each. Transfer the pans to the refrigerator to chill for at least 30 minutes before baking.
Position rack in center of oven and preheat the oven to 300?F.
Bake the shortbread until it is pale golden, 35 to 40 minutes.
Using oven mitts or pot holders, remove the pans from the oven. Cool the shortbread in the pans on a wire rack for 5 minutes. While the shortbread is still very warm, use a sharp knife to cut through the score marks all the way to the bottom of the pan.
Cool the shortbread completely, then carefully remove the wedges from the pans. Store the shortbread in an airtight container, layered with wax paper. Shortbread keeps very well but should be handled gently.
Yield: 16 wedge-shaped shortbread cookies