Chocolate mousse cake is a very rich cake often made with dark chocolate. All of the ingredients are high in calories. The chocolate has a large number of calories from both fat and sugar, while the butter is high in fat.

Chocolate mousse cake recipes vary. However, a BBC Food recipe lists the key ingredients as 10 1/2 oz. of dark chocolate, 5 oz. of unsalted butter, 2 oz. of caster or baker's sugar and six eggs. The recipe serves six people.

According to the USDA's MyFood-a-pedia, 10 1/2 oz. of dark chocolate contains 1,499 calories. Unsalted butter contains 1,020 calories per 5 oz. Caster sugar equals 227 calories per 2 oz. Six large eggs contain 462 calories.

The total calories in a large chocolate mousse cake number 3,208. That means a single portion of chocolate mousse cake has around 534 calories. Many of those calories are from carbohydrates and fat. For example, 1 3/4 oz. of dark chocolate contains almost half the daily suggested 50 g limit of sugar, according to CalorieLab. Around 30 percent of dark chocolate is made up of fat.