Preheat oven to 350F/175C. Line an 8×8-inch baking pan with parchment paper or aluminum foil, leaving enough overhang on the sides for lifting the brownies out after baking.
In a large heat-proof bowl, place chocolate and butter. Heat in the microwave in 20-second intervals, stirring in between each interval, until melted (or set it over a saucepan of simmering water, stirring occasionally). Mix in molasses and sugars. Add eggs, one at a time, and whisk just until combined. Add flour, salt and spices and fold, using a rubber spatula, just until combined. Do not overmix.
Pour batter into prepared pan and smooth the top. Bake for 20 minutes, until the edges are set but the center is still soft. Be careful not to overbake. A toothpick inserted into the center of the brownies should come out with moist crumbs, not dry. Let brownies cool completely on a wire rack, then refrigerate until set for easy cutting. Lift brownies out of the pan using parchment paper and cut into squares.
Store brownies at room temperature or in the fridge in an airtight container for up to 3 days. Brownies can also be frozen for up to 2 months. Thaw them on the counter, still covered, or overnight in the fridge.