In a small microwave-safe container, sprinkle the gelatin powder into water from ◎ and let soak.
Put the ingredients from ★ into a microwave-safe bowl, and microwave it a little over a minute at around 500W without plastic wrap. Dissolve the coffee by heating without letting it boil.
Add sugar (if desired) to soaked gelatin from Step 1 and dissolve that into the bowl while it's still hot. Let it cool after blending it all together. Put the bowl in the refrigerator and let it chill for 20-30 minutes.
If the mixture becomes too cool and hardens, you can either keep it at room temperature, or blend thoroughly with a whisk until it turns back into the mixture like the photo.
In a different bowl, put the chilled heavy cream and blend until it thickens to about the same thickness as the mixture in Step 4.
Combine the mixtures from Step 4 and 5, and mix evenly. Pour equal amount into the two containers.
Put the mousse back in the refrigerator, and it's done once it's chilled and hardened. Place topping as desired. Roasted nuts, etc. bring about a different texture and make it delicious.
[Note 1] It's easier if you use coffee that dissolves well in cold water too. I also recommend adding a bit of liqueur (rum, Kahlua, etc.) to turn this into a dessert for adults.
[Note 2 ] The main, step-by-step photos were taken when I doubled this recipe and made it using a 200 ml capacity coffee cup.
[Note 3] The coffee bean shaped chocolate I used for my topping is Meiji's "Coffeebeat." It's about 100 yen.
Story Behind this Recipe
1. I got the idea (if my vague memory serves me right) from a cocoa mousse recipe (or it could've been a chocolate mousse, I can't remember), which I used to make a lot from a recipe found in my diet sweets cookbook. 2. Coffee is easier, cheaper, and more convenient to make (than cocoa, chocolate, etc.). These ingredient amounts are what I ended up with, after experimenting many times trying to make it fluffy and creamy, while keeping down the calories..
At Step 3, there's more water than gelatin powder, so I dissolve it in residual heat (in the bowl from Step 2). If this worries you, add gelatin (and sugar if desired) to the bowl after dissolving it completely in a microwave. As a reference, heat it a little over 10 seconds at about 500W without plastic wrap. This recipe will be about 108 total calories (For 2 servings, excluding topping) if you substitute heavy cream and milk with low-fat ones, and the sugar with artificial sweetener (e.g. about 2 teaspoons of Palsweet 1/12). Please refer to this as a guide if you need to make this as low calorie as possible due to dietary restrictions, etc.