Place the butter and chocolate in a heatproof bowl set over a saucepan of simmering water. Don’t let the water touch the bottom of the bowl. Heat until the chocolate and butter are melted, stirring occasionally, then remove from heat and let cool. Whisk in espresso powder, cocoa powder, vanilla, Kahlua, salt and two eggs until combined. Mixture will look slightly curdled-- it's okay.

Separate the remaining four eggs. Whisk the yolks into the chocolate mixture and put the whites in the bowl of a stand mixer fitted with a whisk attachment. Alternatively, you can use a large bowl and a hand mixer. 

Whip egg whites on medium speed until frothy, then slowly add the sugar. Continue to beat until the whites are stiff and glossy, about 5 minutes.

Use a rubber spatula to stir a small amount of whipped egg whites into the chocolate mixture to lighten the batter. Then gently fold the whites into the chocolate mixture in three additions, folding until just mixed. Pour into the prepared pan and smooth the top.

Bake for 35- 40 minutes, or until the cake is puffed and the top of the cake is firm and just beginning to crack. Place the cake on a rack and let cool completely in the pan. The cake will fall in the center and crack more on the surface. 

To un-mold, run a knife along the edge of the cake to loosen it from the springform pan, then carefully release the sides.

Just before serving, make the boozy vanilla whipped cream. In the bowl of a stand mixer fitted with a whisk attachment, beat heavy cream on medium speed until soft peaks form, about 3 minutes. Add powdered sugar, Kahlua and vanilla and whip just until the cream forms stiff peaks. 

Serve cake with a dollop of whipped cream.