Place the chocolate in a medium heatproof bowl. 1/4 fill a small pot with water and place the chocolate bowl on top, ensuring the bowl doesn't touch the water.
Turn the heat to medium and allow the chocolate to melt, stirring intermittently. Just as it begins to melt, add the oil and stir to combine. Once the chocolate has completely melted, take it off the heat and allow to cool a little. Take the bowl off the pot to eliminate the residual heat.
Separate the egg yolks and whites in two clean, dry bowls. Add each clean egg white into your stand mixer or a clean mixing bowl.
Once you have the three egg whites ready, whip them on a medium speed until they are completely fluffy. They should have a firm consistency and you should be able to invert the bowl without the egg white slipping. Set aside.
Now that the chocolate is cooled, add the egg yolks to the chocolate. Stir just to incorporate and no further - you might notice the chocolate begins to seize. Only stir them to incorporate without seizing up the chocolate too much.
Take a big spoonful of whipped egg white and place it into the chocolate mixture. Use a silicon spatula to start folding in the egg white. This first spoonful is sacrificial and aims to loosen up the chocolate, so don't worry too much if it loses fluffiness.
The mixture looks a bit scary and like it will never come together, but I promise it does with time and patience. Once you have incorporated the sacrificial egg white, add the remaining egg white.
Continue folding the egg whites into the chocolate, taking more care to avoid deflating them now. The air in the whipped egg will create an airy mousse, so take your time to fold it in properly. Rushing this step or stirring instead of folding will only ruin your own mousse.
Continue patiently folding until every little chunk of egg white has been incorporated fully. Taste your mousse and add a pinch of fine salt and vanilla to your tastes. If the mousse is not sweet enough, add maple syrup to taste and fold gently to combine.
Transfer your mousse to the glasses or bowls you intend to serve it in. Place into the fridge for 5-10 minutes for the mousse to firm up before serving. I find this time in the fridge seems to make the mousse more airy, even if it looks a bit flat before you pop it in the fridge.
Serve with berries or whatever you fancy. Return leftovers to the fridge and eat within a few days to be on the safe side.