Cake: Preheat the oven to 350 F. Place molasses and baking soda in a stainless steel bowl. Pour in boiling water. Whisk together and allow to cool.
Place butter and brown sugar in a kitchen mixer with a paddle attachment and cream together at medium speed until fluffy. Add egg and ginger. Scrape down the sides of the bowl and mix for another 2 minutes. Transfer to a large bowl.
In another bowl, sift together flour, baking powder, salt, ground ginger and ground cinnamon. Alternating between wet and dry ingredients, stir molasses mixture and flour mixture into creamed butter (finishing with the dry ingredients).
Pour batter into a buttered and floured and parchment-lined 9 x 13-inch rectangular cake pan. Bake in the oven for 25 minutes, then decrease the heat to 325 F and continue to bake for a further 15 to 20 minutes, until done. (Cake is done when a toothpick inserted into the centre pulls out clean.) Remove from the oven and allow to cool or serve warm.
Place milk in a saucepan on medium heat and bring just to a boil. Remove from the heat, but leave burner on.
Whisk together egg yolks and sugar in a bowl, until light and fluffy. Temper egg mixture by whisking in a small amount of the warm milk, then slowly add the rest of the milk.
Strain through a fine-mesh sieve into a clean saucepan and return to the heat, stirring constantly with a wooden spoon. Bring to 175 F on a candy thermometer, or until the sauce coats the back of the spoon. Do not boil or overcook, as it will curdle. Add melted chocolate and ground cinnamon. Remove from the heat and immediately strain through a fine-mesh sieve into a clean stainless steel bowl placed over a bowl of ice to the stop cooking, and allow to cool. Cover and refrigerate until ready to use.
Place 2 tablespoons in the centre of each of eight to ten dessert plates. Arrange a slice of gingerbread cake on top of each serving of crème anglaise.