Preheat oven to 375º F. Generously butter a 9-inch square baking pan.
Measure boiling water into a heatproof 1-cup measure with a pour spout. Add baking soda and stir until the baking soda dissolves. Set aside.
In a large bowl, whisk together molasses, sugar, eggs, and butter until well combined. Mix in spices and salt.
Add flour and stir with a spatula or wooden spoon until the mixture is smooth, about 40 strokes.
Pour in the water/baking soda mixture and stir until well combined.
Add the the batter into the prepared pan; it will be quite liquid. Bake in the middle of the preheated oven until the gingerbread rises and the top springs back when lightly pressed with a fingertip, about 35 to 45 minutes (the gingerbread will bake faster in a metal pan than a glass or ceramic one).
Let the gingerbread rest for at least 20 minutes before slicing and serving. Top with whipped cream or vanilla ice cream and drizzle with lemon sauce.