Prep the pan and oven. Preheat the oven to 350°F (175°C). Line an 8-inch square baking pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Melt the butter. In a medium bowl, melt the butter and let it cool for 3–5 minutes so it doesn’t scramble the egg.

Whisk the wet ingredients. Add brown sugar and molasses to the butter and whisk until smooth and glossy. Whisk in the egg and vanilla until well combined.

Combine the dry ingredients. In a separate bowl, whisk flour, ginger, cinnamon, cloves, nutmeg, baking soda, and salt. This prevents spice clumps and ensures even rise.

Bring it together. Add the dry mixture to the wet ingredients and stir with a spatula until just combined. If using mix-ins, fold them in now. The batter will be thick and sticky.

Spread and smooth. Transfer the batter to the prepared pan. Use a spatula or damp fingertips to press it evenly into the corners. Even thickness helps prevent dry edges.

Bake. Bake for 16–20 minutes, until the center is set and a toothpick comes out with a few moist crumbs. Do not overbake or they’ll turn cakey.

Cool completely. Place the pan on a rack and cool before glazing or slicing. Warm bars can tear and the glaze won’t set.

Make the glaze (optional). Whisk powdered sugar, 1 tablespoon milk, vanilla, and a pinch of salt until smooth. Add more milk drop by drop to reach a pourable consistency. Drizzle over cooled bars.

Slice and serve. Lift the slab out using the parchment and cut into 12–16 squares. Enjoy as is or with a dollop of whipped cream.