Preheat the oven to 350 degrees F. Line the bottom of a spring form pan with parchment paper. Clamp the ring into place and trim the paper around the edges. Spray the sides of the pan with unstuck cooking spray.
In a large bowl whisk the first 7 dry ingredients together.
Whisk the eggs, milk, oil and vanilla together separately. Then slowly add the wet mixture into the dry mixture, whisking until smooth. You can use an electric mixer if you like.
Pour the batter into the prepared pan and bale for approximately 30 minutes—until a toothpick inserted into the middle comes out clean.
Allow the cake to cool in the pan. Then trim the top with a serrated knife for a flat surface.
Fit a sheet of parchment paper around the cake to form a large collar. Place the spring form rim back place to hold the collar.
For the Mousse:
Pour the cream into the bowl of an electric mixer. Using the whisk attachment, whip until firm peaks form. Place in an air-tight container and refrigerate for at least one hour.
Wash and dry the bowl, then place 6 egg yolks in the mixer bowl and whip until a light and frothy—5 minutes.
Meanwhile, add 6 tablespoons sugar, 3 tablespoons corn syrup, 3 tablespoons of water, 1 tablespoon coffee granules, 2 teaspoons of vanilla and a pinch of salt in a small saucepan. Boil to a boil for about one minute, then remove from heat.
With the mixer running, carefully pour the hot sugar mixture into the egg yolks. Increase the speed and whip another 3-5 minutes until thick.
Then slowly pour the melted dark chocolate into the mixture. Scrape the bowl.
With a spatula, gently fold HALF the cold whipped cream into the chocolate mixture.
Once combined, carefully spoon the chocolate mousse onto the center of the cake layer. Spread evenly and refrigerate.
Repeat steps 6-13 with the remaining ingredients. Take note that the milk chocolate mousse has 1 extra egg yolk and less corn syrup, water, and vanilla. Milk chocolate contains more moisture, so the mousse needs to be adapted so it isn’t too loose to stand up.
Spoon the milk chocolate mouse over the dark chocolate. Spread evenly. Refrigerate for a minimum 4 hours or over night to set.
Once set, carefully remove the paper collar. Keep chilled until ready to serve.