01 - Preheat your oven to 350°F (175°C). Line a 9x9-inch (23x23 cm) baking pan with parchment paper, ensuring an overhang for easy removal.
02 - In a large mixing bowl, whisk together the all-purpose flour, ground ginger, ground cinnamon, ground nutmeg, ground cloves, baking soda, and salt until thoroughly combined.
03 - In a separate bowl, whisk together the melted unsalted butter, packed brown sugar, molasses, milk, large egg, and vanilla extract until the mixture is smooth and well-emulsified.
04 - Pour the wet ingredients into the bowl with the dry ingredients. Stir gently until just combined; do not overmix.
05 - Carefully fold in most of your chosen mix-ins into the batter, reserving a small handful to sprinkle on top later.
06 - Spread the batter evenly into the prepared baking pan. Sprinkle the reserved mix-ins over the top and gently press them into the batter.
07 - Bake for 22 to 25 minutes, or until a toothpick inserted into the center comes out mostly clean. A few moist crumbs are acceptable.
08 - Allow the bars to cool completely in the pan on a wire rack before cutting them into 16 individual portions.