Preheat your oven to 300C/150F and trace two 6″ circles onto parchment paper. In the bowl of a stand mixer, whisk 3 egg whites until frothy and stiff. Add the brown sugar and continue whisking until the sugar has dissolved into the egg whites. Divide your meringues even between your two circles and spread it out to the edges of your pen/pencil marks. Using your spatula, create a shallow indent in the top of your meringues (this will help hold the filling). Place your pavlovas in the oven and immediately reduce the heat to 250F/120C. Bake for an hour, then turn off the heat and allow the pavlovas to cool in the oven (about an hour).
Once the meringue has cooled, melt 2 of your dark chocolate bars and divide them evenly between your two meringues. Spread the chocolate evenly across the top. The remaining bar can be used for garnishing by coarsely chopping it, grating it, or creating chocolate curls.
In the bowl of a stand mixer over the a pot of bowling water, whisk together the remaining 6 egg whites and the granulated sugar. Once the mixture is hot, remove the bowl from the pot of boiling water, and whisk on high until cooled and stiff peaks form. Add the vanilla and continue whisking until just combined. Spread the marshmallow frosting over each layer of pavlova, staking as you go.