Preheat the oven to 325 degrees F and grease and line an 8-inch square baking tin with parchment.
Sift the flour, ginger, allspice, salt and bicarbonate of soda into a bowl. In another bowl, cream the butter and brown sugar until very soft. Beat in the eggs one by one, then mix in the treacle. Gently fold in the sifted dry ingredients.
Pour the mixture into the prepared tin and bake in for 50 to 60 minutes, or until the top springs back when pressed.
Cool in the tin, remove and store for a few days in an airtight tin before eating. The cake can be served in slices spread with butter, or with clotted cream and stem ginger.