Before you even open the cream carton, pop your mixing bowl in the freezer for about 15–20 minutes. I know it feels like an extra step, but a cold bowl keeps the cream from warming up while you whip it. Warmer cream takes longer to whip and won't hold its shape as well once it's done.
Pour the cold heavy cream into your chilled bowl and start whipping on medium-high speed. Keep going until the cream looks frothy and just starts to thicken, almost like a loose pancake batter. This usually takes me about a minute with an electric hand mixer, so don't walk away from the bowl.
Sift the cocoa powder and powdered sugar directly over the whipped cream. I like to give it a gentle stir with a spatula first so the mixer doesn't blast powder everywhere. Once it's roughly combined, switch the mixer back on until soft peaks start forming.
If you're using almond or vanilla extract, this is the moment to add it. Pour it in once soft peaks have formed, then keep whipping until the mousse holds firm, stiff peaks. This is the texture you're aiming for thick enough to hold its shape on a spoon.
Stop the mixer the second you see stiff peaks form, because chocolate mousse can go from perfect to grainy in seconds. I always finish the last few strokes by hand with a spatula so I don't accidentally overwhip it into chocolate butter.
Spoon the mousse into a piping bag or a ziplock bag with the tip cut off, then pipe it into small serving cups or glasses. You can serve it right away for a soft-set mousse, or chill it in the fridge for 20–30 minutes if you prefer a firmer texture.