Heat the oven to 350°F (175°C). Line an 8-inch or 9-inch square pan with parchment paper, leaving enough overhang on two sides to lift out the cooled bars.

For especially plump raisins, place them in hot water for about 10 minutes. Drain them thoroughly and pat them dry before adding them to the batter.

Beat the softened butter and packed brown sugar until smooth and slightly fluffy. Add the molasses, egg, and vanilla, then beat until combined; the mixture may look slightly curdled, which is normal.

In a separate bowl, whisk together the flour, baking soda, salt, cinnamon, ginger, and cloves or allspice. This distributes the leavening and spices evenly throughout the finished bars.

Add the dry ingredients to the molasses mixture and stir only until no visible streaks of flour remain. Avoid overmixing, because too much stirring can make the bars less tender.

Fold in the raisins and chopped walnuts or pecans, if using. The dough will be thick and sticky, so scrape the bowl well to incorporate the flavorful molasses mixture.

Press the dough evenly into the prepared pan using a lightly greased spatula or dampened hands. Smooth the surface so the center and edges bake at the same rate.

Bake for 20 to 25 minutes, until the edges are set but the center still looks slightly soft. Cool the slab in the pan for at least 30 minutes before lifting and cutting; cool completely for the cleanest slices.