Preheat oven to 350ºF. Grease a 13"x18" standard sheet pan.
In a medium bowl, whisk together the brown sugar, molasses, water, butter, baking soda, and salt. Let it cool for 10 min.
Meanwhile, whisk the rest of the dry ingredients together (flour, ginger, cinnamon, nutmeg, cloves, allspice).
Whisk the egg into the wet ingredients.
Stir the flour mixture into the wet ingredients until well combined.
Pour into the prepared sheet pan and bake for 15-20 min, or until the cake springs back to touch.
Cool for about 5 min before removing from the pan and cutting into 4 pieces, half lengthwise and half widthwise.
To make the frosting, in a bowl that fits over a pan as a double boiler, whisk the egg whites, sugar and cream of tartar until combined. Using an electric whisk, continue beating the mixture over medium heat until the frosting thickens to medium peaks.
Remove from heat. Add maple syrup and cinnamon. Continue whisking till medium-stiff peaks form. Cool for 10 min.
To assemble, start with one layer of cake. Spread 1/4 of the frosting on top and repeat until all layers are completed. Dust with cocoa powder or blow torch the top. Cool to room temperature before slicing for even pieces. Enjoy!
Notes
For molasses, you can mix and interchange with honey and brown rice syrup as long as you come up with 2/3 cup of it!
A double boiler is a bowl or pot on top of another pot with simmering water. It's a way to evenly heat ingredients. Make sure the top bowl/pot doesn't touch the water below.