Begin by making the chocolate mousse frosting. Combine the chocolate and water in a microwave safe bowl. Microwave on 30 seconds intervals, stirring after each, until just melted. Once the chocolate is melted whisk in the four egg yolks. Don't stop whisking until they are completely combined. Set aside. In a clean, dry bowl of a stand mixer fitted with the whisk attachment, combine the egg whites and the salt. Whip on medium-high speed until stiff peaks form. They will be thick and smooth. This took me about five minutes. Add one third of the whipped egg whites to the bowl with the chocolate and fold gently to lighten up the mixture. Fold in the remaining egg whites gently with a spatula only until there are no visible streaks left. Be as gentle as possible, you dont want to deflate the egg whites. Cover the bowl and place in the freezer for at least one hour while you bake the cupcakes.
Heat oven to 350°F. Line a cupcake tin with liners.
Stir together flour, sugar, cocoa, baking powder, baking soda and salt in large bowl. Add eggs, buttermilk, oil and vanilla. Beat on medium speed of mixer for two minutes. Beat in Kahlua. Evenly scoop batter into prepared muffin liners.
Bake 12 to 15 minutes or until wooden pick inserted in center comes out clean. Allow to cool completely before filling and frosting.
While the cupcakes cool make the filling and chocolate Kahlua sauce. For the sauce, whisk together the heavy cream, Kahlua, and salt in a small bowl. Set aside. Combine the brown sugar and water in a small saucepan over medium-high heat. Bring the mixture to a boil. Once it begins to boil watch closely and whisk until the mixture starts to smell like caramel and turn a golden color, and then to a light brown. This will take about 3-5 minutes. When you see that it has started to turn brown, slowly pour in the heavy cream mixture, whisking constantly and quickly. The sugar will harden around the whisk, don’t worry, it will melt. Just keep whisking! Continue to heat over medium-high until mixture has thickened and is a light caramel color, about 5 minutes more (don't let it get too thick, it will thicken as it sits). Remove from the heat and stir in the chocolate. Set aside.
Pour the cream into a bowl of a stand mixer. Whip on medium high speed for about two minutes, then turn the speed to high and add the powdered sugar, Kahlua and vanilla. Whip until stiff peaks form. Place in the fridge until ready to use.
To assemble the cupcakes Use a small paring knife to cut a cone-shaped piece from the center of each cupcake. Fill the hole with the whipped cream filling. Transfer the Chocolate Mousse Frosting to a pastry bag fitted with a tip. Pipe frosting (make sure it is cold) into a swirl on each cupcake (you can also just spread the frosting on with a knife). The mouse may or may not hold it's shape, it depends on how cold and stiff it is. Don't worry if the mousse does not hold it's shape, it's still delicious! Sprinkle with chopped Snickers and drizzle the chocolate Kahlua sauce over the top, if desired.
Chocolate mousse adapted from [Two Peas & Their Pod| http://www.twopeasandtheirpod.com/easy-chocolate-mousse/]