Cultural Awareness Day,... how did we miss that?
Double cream weighs in at 48% butterfat, or at least 6% higher than the highest-fat cream available in the United States - heavy cream or heavy whipping cream, which is at least 36% to 40% fat (the wonderful New England dairy from which we buy our cream produces a 42%-fat cream, although it doesn't brag about it). British recipes call for double cream because they can - it's available and adds lots of richness to your dish. Will your cake flop if you use heavy cream? Almost certainly not.
Can you get a higher-fat cream to use in the recipe? Yes, with a little effort on your part. The first step is to prepare a homemade version of crème frâiche. Combine 2 tablespoons of buttermilk or sour cream with 2 cups of heavy cream. Heat the mixture to body temperature in a pan, and then let the mixture sit in a non-reactive container (stainless-steel, glass, or ceramic) in a warm place for 24 hours or so, until it thickens.
Step 2 is to line a strainer with a triple thickness of cheesecloth or a wet napkin, fill it with the crème frâiche, and suspend that over a container in the refrigerator. The whey will drain out of the cream, leaving it with a higher fat content. By the time about a quarter of a cup of whey drains from your 2 cups of crème frâiche, you will have a fat content around 48% to 50%. It will taste a little more acidic than fresh cream, and you will have to judge for yourself whether that might negatively affect the flavor of your cake.
Everyone at Cultural Awareness Day will be amazed at the Irishness of your chocolate cake (although some idiot will probably wonder why it's not green).