Butter the bottom and sides of a 9 inch square pan and line the bottom with parchment paper. Dust with cocoa powder and tap out the excess.
In a heatproof bowl, combine the butter and both chocolates. Create a double boiler by setting the bowl over a pot of simmering water, stirring occasionally until it's melted. Remove the bowl from the heat.
PRO-TIP: You do not need a lot of water for this. Just enough so it creates steam and so it does not all boil away.
In a stand mixer fitted with your whip attachment, whisk together the egg yolks and 1/2 cup of sugar on high speed until it goes pale, increases in volume and it leaves a defined ribbon on the surface when you lift the whisk (about 5 minutes).
Fold in the melted chocolate until fully incorporated.
In a clean and dry bowl, whisk the egg whites and salt on a low speed until the form soft, wet peaks. Gradually beat in the remaining 1/2 cup of sugar and continue whisking at high speed until the whites form stiff peaks.
Scrape the batter into the pan and gently smooth the top.
Bake for about 35 minutes (It will still have a slight wobble on the top). Look for the cake to rise and formed a cracked top.
Let cool for 15 mins.
Invert the Chocolate Pave onto a plate or a cake board and remove the parchment paper. Re-invert onto your serving plate. Dust with icing sugar if you wish.