Preheat oven to 425°F. In a large bowl, add the flour, baking powder, sugar, cocoa powder, and salt. Whisk together to combine.
"Cut" in the butter with a pastry knife or your fingers. Mix until the butter is in pieces no bigger than peas, and the mixture looks like wet sand.
Add the milk and mix together with a wooden spoon until the flour is wetted. Pour out onto a lightly-floured, flat surface and pat into a rectangle, sort of like pie dough. Try not to over-work the dough.
Lightly flour a rolling pin and the top of your dough and roll the dough out to about 1/2 inch thick. Use a round cookie/biscuit cutter, and cut out the shortbread biscuits. I used a 2.5 inch round, but you can use whatever size you want.
Bake in the preheated oven for 12 minutes. Allow to cool before assembling.
For the Mousse:
Chop the chocolate into fine pieces, and melt it with the butter in a heat proof bowl either in a double boiler or in the microwave. Whisk together if separated. Allow to cool slightly.
Add the 4 egg yolks to the butter and chocolate mixture, one at a time, whisking thoroughly to incorporate. Set aside.
Using a stand mixer or a hand mixer, whip the egg yolks with the cream of tartar until foamy. At this stage, slowly add the sugar in. Continue whipping until you reach stiff peaks.
Gently fold the meringue into the chocolate mixture. Fold until no white streaks remain but be careful not to knock the air out of the meringue. Set aside.
In a clean bowl, use the stand mixer or hand mixer and beat the whipped cream and vanilla extract to stiff peaks. Fold the whipped cream into the chocolate and meringue mixture. The end result should not have any streaks of white, and should be a light brown color. It should have a lot of air in it.
Separate the mousse into individual serving containers or just leave it in the bowl. Cover the containers holding the mousse with plastic wrap and refridgerate for at least 2 hours.
For assembly:
Once the mousse has been in the refridgerator for at least 2 hours and the shortbread is cool, split the shortbread in half like a hamburger bun. Place a hefty dollop of mousse on the bottom half of the shortbread and top with sliced figs. Place the top of the shortbread on, making a sort of sandwich, and enjoy!