Preheat your oven to 325 F degrees. Coat a 9x5-inch loaf pan with cooking or baking spray. Set aside.
In a medium bowl whisk together the flour, baking soda, ginger, cinnamon, allspice and salt. Set aside.
In the bowl of your mixer, add the butter and brown sugar, and beat on high speed using your paddle attachment, for about 1 minute. Add the egg and vanilla extract and beat together for another 30 seconds or until well blended, scrape down the sides of the bowl as necessary.
Meanwhile in another smaller bowl whisk together the molasses with the hot water.
Add a third of the flour mixture and beat on low until well incorporated, then 1/3 of the molasses/water and continue beating. Repeat this a couple more times, alternating between flour and molasses mixture. Please note that the batter will be quite thin.
Pour the batter into the prepared loaf pan and bake for about 50 minutes or until baked through. To test if the loaf is baked through, insert a toothpick in the center of the loaf, and if it comes out clean, it is done.
Allow the bread to cool until it comes to room temperature in the pan, before removing it from the pan.
To make the icing, add the powdered sugar, cream cheese, vanilla extract and lemon juice to the bowl of your mixer and beat on high using the whisk attachment until smooth. Spread the icing over the loaf, the icing should be quite thick, so it should be easy to spread.
Melt the the red and green candy melts, if using, in the microwave. I usually microwave them for 1 min, stir then microwave for another 30 seconds and they should be melted. Drizzle the chocolate using a teaspoon over the gingerbread loaf.
Cut into slices and serve.