Place the butter and molasses in a small saucepan and bring to a boil over medium-high heat, making sure to stir it constantly. Once it is boiling, transfer to the bowl of a stand mixer and allow it to cool for 5 minutes. Add in the egg, heavy cream, sugar, lemon juice, and orange zest and mix with the fitted paddle attachment.
If using a gluten-free flour blend, I recommend the King Arthur Measure for Measure blend. You can generally use the same amount, but gluten-free flour tends to be more dense and the moisture in your cake can vary depending on your altitude. It may require 3 1/2-4 cups depending on where you live and which blend you use. A good rule to determine proper cake batter consistency is to make sure the batter is puffy and thick, not watery or runny.