Make the blondies: In a medium bowl, add the slightly cooled brown butter, brown sugar, and molasses and mix until combined. Then add the egg and vanilla extract and mix until completely combined. Then sift in the flour, baking soda, salt, cinnamon, ginger, nutmeg, and all spice, mix until just incorporated. Do not over mix.
⅔ cup (133g) brown sugar, packed, 1 tablespoon (20g) molasses, unsulphured, 1 large egg, 1 teaspoon vanilla extract, 1¼ cup (150g) all purpose flour, ½ teaspoon baking soda, ½ teaspoon salt, 1 teaspoon ground ginger, ¾ teaspoon ground cinnamon, ⅛ teaspoon ground allspice, ⅛ teaspoon ground nutmeg