In a stand mixer fitted with the paddle attachment, cream together the shortening, sugar, molasses, and egg until smooth.

In a medium bowl, whisk together the our, baking soda, salt, ginger, and cinnamon until combined. Reduce the speed to low and add about a third of the our mixture, followed by half of the hot water. Scrape down the sides of the bowl and add half of the remaining flour. Pour in the remaining water and the rest of the flour.

Transfer to the prepared baking pan and bake for 35 to 40 minutes, until a toothpick inserted in the center comes out clean.

Make the lemon sauce by stirring together the sugar, cornstarch, salt, nutmeg, and half-and-half in a small saucepan over medium heat. Continue to cook until bubbly, thickened and smooth. Reduce heat to low and continue stirring occasionally while preparing the next steps.

In a small bowl, beat the egg yolks. Remove the saucepan from heat and slowly stir about 1/4 cup of the sugar mixture with the egg yolks.

Transfer back to the saucepan and heat on medium until boiling. Cook for another 2 minutes. Off the heat and stir in the butter and lemon juice.