I'm sharing because I've tried very hard (like for 3 tries) to make an awesome, stiff, and easy chocolate mousse for cake fillings. I think I've found what works. Here goes.

1 cup dark chocolate chips, blocks, what have you—but not unsweetened bakers chocolate. Dark chocolate chips work here.
4 TBSP butter (salted, unsalted, que sera, sera)
1 tsp vanilla, good vanilla or rum or other dark liquor. Not Drambuie though, that's for your coffee. Heathen.
2 cups full fat allthefuckingway. Heavy cream.

Melt chocolate carefully so as not to scorch. Stir in butter and vanilla (or what the hell ever).
Temper 1/2 cup of the cream. This means to add a small amount of the warm chocolate to some cream which should be very cold. Mix together and then put all cream plus this tempered mix into a stand up mixer or large bowl to be used with a hand mixer. If you are mixing this, literally, by hand then brothah/sistah you've earned this shit. Just eat it by the spoon when the stiff peaks form. Don't mix much beyond stiff peaks, otherwise the fats become grainy and you're at risk of making chocolate butter. Not altogether bad, but not what we were going for. What I love about this recipe so far, it taste like a less sweet version of the 12-egg French mousse you get served in pretentious glass bowls; yet it is nicely stiff and holds between cake layers.
Hope someone else enjoys this too.