Separate the eggs, placing the yolks in the bowl of your mixer and the whites in a small bowl.

Melt the chocolate chips and butter in a large heatproof bowl set over a pan of barely simmering water, and let cool slightly.

Using the whisk attachment beat the egg yolks till just blended and, with the mixer still running, slowly add 1/2 cup of the sugar. Beat till well blended. Scrape in the chocolate mixture (do not clean out the bowl) and beat again till well blended. Pour the mixture back into the bowl that the chocolate was in and set aside.

Clean the mixer bowl and attachment and add the egg whites and 2 tablespoons of the sugar. Beat on high till stiff peaks form and fold, in 3 additions, into the chocolate mixture.

Pour the batter into your prepared pan and bake till the cake is puffed up and a cake tester inserted into center comes out with moist crumbs attached, about 30 minutes. Cool the cake in the pan on a wire rack for 10 minutes. Use a small knife to cut around the edge of the cake and gently loosen the pan. Invert the cake onto the rack, peel off the parchment paper and let cool completely. Wrap and chill overnight.

Beat the heavy cream with the remaining 2 tablespoons of sugar till stiff peaks form. Use a small angled spatula to spread the whipped cream over the top of the cake and dust with the cocoa powder (can be made 4 hours ahead – cover and chill).