Preheat the oven to 325F. Cut a couple of circles of waxed paper and place them in the bottom of two 8" or 9" round cake pans - trust me it's worth the extra 2 minutes of effort, since you have to be gingerly with their removal on account of the meringue on top. Spray them, lined with waxed paper, with nonstick spray.
In a large bowl, beat the butter and sugar for a few minutes, until fluffy. Add the eggs one at a time, then the cocoa mixture and vanilla. Add the flour, baking powder and salt and beat on low just until blended. Divide between the prepared pans and smooth the surface.
To make the meringue, beat the egg whites with an electric mixer until soft peaks form; gradually beat in the sugar and continue to beat until stiff and glossy. Spoon the meringue on top of the cake batter and gently spread into a circle, leaving about an inch clear around the edge to allow it to expand as it bakes.
Bake the cakes for 35-40 minutes, until the meringue is golden and if there is some cake peeking through, it's springy to the touch. Let cool in the pans until just warm, then gently turn them out onto your hand, peel off the waxed paper and cool them on a wire rack.
To assemble the cake, whip the cream (adding a bit of sugar if you like) and spread it on one cake layer; top with the other.
If you like, sprinkle hazelnut praline over top.