Mousse is meant to have an ethereal, light consistency; in fact, its quality is gauged by its airiness. But tastes differ, and cooks who favor a rich, pudding-like consistency often find classic French mousse a bit light on the palate. Mousse's thickness correlates with its airiness -- the thicker the consistency, the less aerated the mousse -- so you have to sacrifice a little fluffiness when you thicken it.
Before You Begin
Look at other mousse recipes before adding a thickening agent to your current mousse. The widely agreed-upon classic French chocolate mousse calls for egg whites, sugar, cream and chocolate, which essentially makes a meringue with chocolate folded into it. Another well-known recipe, published in "Mastering the Art of French Cooking" by Julia Child, calls for equal parts butter and chocolate in addition to egg whites and yolks, creating a dense mousse almost on par with pudding in terms of consistency.
If you're happy with your recipe, and you've considered an alternative, take it slow and experiment with thickeners from light to heavy.
Nothing thickens mousse as gently as a liaison. Made with egg yolk and cream, the liaison does not add anything to mousse that isn't already there -- it just changes the ratio a bit in favor of thickness.
Bring a half-full pot of water to a boil. Place a stainless-steel bowl over the saucepan and add 1 egg yolk and 3 tablespoons of cream for every 2 cups of finished souffle (a basic souffle recipe yields about 4 cups of finished souffle). Mix the cream and yolks until it has no traces of yellow. Heat the liaison until it reaches 160 degrees Fahrenheit, whisking occasionally. Mix the liaison with the main ingredient (whisk it into the melted chocolate when making a chocolate souffle). Fold the egg whites and whipped cream (the cream called for in the recipe) into the thickened chocolate.
Egg yolks provide about twice the thickening power of a liaison and work best in souffles that already contain them. If you're making a souffle that calls for egg yolks, add 1 additional egg yolk for every 1 cup of finished souffle.
Bring a pot half filled with water to a boil in the stove. Add 1 egg yolk to the stainless-steel bowl for each cup of finished souffle. A basic recipe yields about 4 cups of finished souffle. Whisk the egg yolks until combined and heat them until they double in size, or until they reach 160 F. Combine the egg yolks with the main ingredient.
Consider cornstarch the "nuclear option" for thickening mousse. Cornstarch, when used indiscriminately, can turn a souffle into pudding instantly. If thickening chocolate, add the cornstarch slurry to the melted chocolate. For other types of souffle, add the cornstarch slurry to the main ingredient; then heat it until it thickens. Then, allow the main ingredient to cool to room temperature before adding the souffle.
Mix 1/2 tablespoon of cornstarch with 1/2 tablespoon of cold milk for every 1 cup of finished souffle. Mix the cornstarch slurry into the main liquid ingredient. Heat the main ingredient over a double boiler until it thickens, or for 5 to 7 minutes.
If you're thickening a savory mousse, such as salmon or avocado, simply fold in more of the main ingredient until it reaches the desired consistency. For example, in a basic salmon mousse recipe, which calls for 4 ounces of smoked salmon, add 6 ounces of salmon. Puree the gelatin along with the salmon as instructed in the recipe.
You can also use 25 percent more gelatin to stiffen a savory mousse. A basic savory mousse recipe calls for 1 teaspoon of gelatin to 3 tablespoons of water. To moderately thicken the mousse, use 1 1/2 teaspoons of gelatin and 4 1/2 tablespoons of water.