Preheat oven to 350°F (175°C). Grease an 8x8-inch baking dish or line it with parchment paper.

In a small saucepan, simmer raisins in water for about 5 minutes until plump. Remove from heat and let cool slightly.

In a mixing bowl, cream together butter and brown sugar until light and fluffy.

Mix in the egg and vanilla extract until well combined.

In a separate bowl, whisk together flour, baking soda, salt, cinnamon, nutmeg, and cloves.

Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Fold in the raisins (with any remaining liquid) and walnuts (if using).

Spread the batter evenly into the prepared baking dish.

Bake for 25 minutes, or until a toothpick inserted in the center comes out clean.

Let cool in the pan for 10 minutes before cutting into bars.