Preheat oven to 350°F (175°C).
Soak the raisins in 3 tablespoons of rum for 20 minutes while you prepare the other ingredients.
Sift together the flour, baking powder, baking soda, ginger, nutmeg, allspice, and salt in a large bowl.
In a food processor or mixer, cream together the softened butter, dark brown sugar, and granulated sugar until light and fluffy.
Add the remaining 3 tablespoons of rum, vanilla extract, eggs, mashed bananas, and sour cream to the creamed mixture and process until smooth.
Add the banana mixture to the dry ingredients and stir with a wooden spoon until just combined.
Drain the raisins and fold them into the batter along with the chopped walnuts, if using.
Grease and flour your loaf pan(s). For 5 mini loaf pans (5 3/4" x 3 1/4" x 2"), divide the batter evenly and bake for 35–45 minutes. For one standard 9" x 5" loaf pan, bake for 55–65 minutes, or until a tester inserted into the center comes out clean.
Let cool in pans for 10 minutes, then remove and cool completely on a wire rack.