Method

First, melt the chocolate with the butter and instant coffee powder on a double boiler. TIP: Use a big bowl to melt the chocolate because everything else will finally get added to it. Let it cool to almost room temperature.

Now, normally chocolate mousse is made with raw yolks. There is no smell of eggs detected once the mousse is done BUT I don't like the risk of raw egg yolks, so I decided to add a small twist of my own and lightly steam the egg yolks, If you want, you can skip this step and just add the raw yolks into the cooled melted chocolate and continue with the rest of the recipe.

Make sure you use the freshest eggs you can get to be on the safe/healthier side. But if you're like me and hesitate over the possible side effects of raw yolks, this is what you should do.
Prepare a dish with some icy cold water and have it ready.

Put the egg yolks plus 1½ tsp. sugar and a drop of vanilla in a bowl and again put this bowl over a double boiler and whisk the ingredients together, letting the steam from the pot below warm the egg yolks. Once the color of the yolks turns pale yellow(about 2 minutes),remove from the heat and place the bowl in the dish of icy water whilst continuing to whisk as to cool the yolks quickly.Then add the yolks into the melted chocolate while whisking or stirring together until they are incorporated.

Be careful not to drip water into the chocolate, make sure you wipe the bowl that was in the icy water so nothing drips. (If the chocolate seizes/clumps up/separates, don't worry just add a tsp. or few drops at a time of cold milk and continue whisking until it is smooth and glossy again, don't add too much milk, just a few drops and keep mixing until it smoothens out, it normally doesn't take long)
Set this mixture aside.

Whip the egg whites with the cream of tartar or pinch of salt (it helps the whites hold their shape) until soft peaks form, then add 1½ tbsp. of sugar and continue whisking until peaks are stiff but still shiny and glossy. Set aside.

Working fast, wash and dry the beaters, then beat the whipping cream with the remaining 1½ tbsp. of sugar and vanilla until stiff. TIP: Reserve a bit of the whipped cream to be used for decorating the mousse later if you want.

Now add a third of the whipped egg whites into the chocolate mixture, fold it in, then add a third of the whipped cream...fold it in. Continue this way until both the whites and the whipped cream are all folded into the mixture. Do NOT over-fold otherwise you will lose the light and airy quality. Use a light hand and only fold a few times with every addition to combine. Add crushed oreo and fold all together.

Layer oreo into a bowl or shots, pour mousse into bowl or individual sized bowls. Cover with cling and refrigerate for at least 4 hours but preferably overnight to set and chill completely.

When serving, add a small dollop from the reserved whipped cream on every bowl and maybe some chocolate shavings, Oreo or fruit.
Enjoy! :)