Preheat the oven to 375 degrees F and line a 6-inch cake pan with parchment paper, leaving the edges hanging over the sides for easy removal. 

In a medium size bowl, add the chocolate and butter and place over a pot of simmering water. Stir frequently until fully melted and smooth. 

Once the chocolate, butter mixture is melted, remove the bowl and whisk in the cocoa powder, espresso powder, and vanilla. Set aside to cool.

Meanwhile, in a separate medium size bowl, beat the egg whites until foamy, then gradually add in the sugar. Continue beating until stiff peaks form. 

Once the chocolate mixture has cooled, whisk in the egg yolks one at a time until fully combined. 

Lastly, gently fold in the meringue in 3 additions, careful not to over-mix, but mix just enough so there are no steaks of meringue left. 

Gently pour the batter into the prepared cake pan and smooth out the top. 

Bake the cake for about 40 minutes or until it has risen up like a soufflé, formed cracks on the top, and has a very slight wobble. 

Allow the cake to cool completely in the pan. As it cools it will fall and form more cracks. 

Gently remove the cake, then sprinkle with powdered sugar and enjoy!