Preheat the oven to 350°F with a rack in the center. Line an 8" square pan with a parchment sling and lightly grease.

In a medium bowl, whisk together the flour, gingerbread spice, baking powder, and salt.

In a large saucepan or microwave-safe bowl, heat the butter over medium-low or microwave for 45 to 60 seconds, covered, on 50% power, until just melted.

Whisk the molasses and brown sugar into the butter. Add the egg and vanilla and whisk until the mixture is shiny and emulsified.

Add the dry ingredients to the wet ingredients and whisk until smooth.

Transfer the batter to the prepared pan and spread it to reach the corners.

Bake the gingerbread bars for 20 to 25 minutes, until set in the center.

Remove the bars from the oven and let them cool slightly in the pan before using the parchment to transfer them to a wire rack. Let cool completely.

To make the frosting: Working in a large bowl with a rubber spatula or handheld mixer or in the bowl of a stand mixer fitted with the flat beater, sift in the confectioners’ sugar and salt, then add the butter. Beat until a thick paste forms.

Add the water and beat until the frosting is smooth and spreadable. Cover until ready to use.

Once the bars are fully cooled, spread the frosting evenly over the top.

Working quickly before the frosting sets, decorate with sprinkles and/or gingerbread spice.

Slice into bars and serve at room temperature.

Store leftover frosted gingerbread bars well-wrapped at room temperature for up to 3 days.