Preheat the oven to 350º. Line the baking sheet with parchment. Draw a 9-inch circle on parchment, then flip the paper over. Set aside.: The initial heat gives the meringue a strong start, while the parchment and marked circle guide your shaping. When the oven is at 350º, you will notice the warm, dry air that helps the meringue form a crisp exterior quickly. Preparing the parchment ahead prevents last minute rushing that can deflate your whites. A common mistake is using a damp or greasy sheet, which prevents proper drying. Make sure the circle is clearly visible through the paper so you keep the meringue within bounds.
Beat the egg whites till soft peaks form. Add salt, then slowly add the sugar, while continuing to beat, until the meringue is stiff and glossy. Sift cocoa powder over the meringue, then add vinegar, vanilla, and chopped chocolate. Fold well till combined.: As you beat the egg whites , listen for the change in sound as they transition from watery to frothy, then to soft peaks that hold a shape but still slightly melt at the tip. Adding the salt early brightens the flavor. When you add sugar slowly, you feel the meringue thicken and become glossy; the whisk sound becomes higher pitched and steadier. Sifting cocoa powder prevents lumps and gives even color. Folding in chopped chocolate introduces little pockets of richness. A frequent error is adding sugar too quickly which leaves a gritty texture. If you feel any graininess, continue beating until fully dissolved.
Pile the meringue onto the baking sheet, keeping it within the 9-inch circle. Smooth the top and edges with an offset spatula.: Moving the glossy, cloudlike meringue onto the paper, you will feel its weight, and smoothing should be done gently to keep the interior airy. Use the offset spatula to create slightly higher edges, which helps the center remain a bit marshmallowy when baked. Visually, the surface should be even, with no shiny wet spots. A common slip is overworking the meringue while smoothing, which can deflate it; always use light, decisive motions.
Place in oven and immediately decrease the temperature to 300º.: Dropping the oven to 300º after the meringue goes in creates a slow, even drying that prevents browning and cracking excessively. You will hear almost nothing once the meringue begins to set, but visually the exterior will become matte and begin to firm. If you leave the oven at too high a temperature, the outside will overcook while the inside remains underdone. Avoid opening the oven during the initial minutes as it can cause uneven surface formation.
Bake for 1-1/4 hours till dry on edges and top. Pavlova will appear cracked.: During this long, gentle bake, the edges should feel dry and slightly crisp to the touch, and the top will matte and often show cracks, which is normal and attractive. The aroma is faint and sweet, with a whisper of cocoa. If it still feels tacky after the time, give it a bit longer in small increments. A mistake to watch for is underbaking, which results in a soggy shell; prevent this by testing the edges for dryness.
Turn off the oven, open the oven door, and allow it to cool completely.: Cooling slowly in the warm oven prevents sudden temperature shifts that make the meringue collapse. As it cools, it will contract and the top may crack more, which is expected. You will notice a dry, crisp sound when lightly tapped. A common error is removing the pavlova while it is still warm, which can lead to collapse, so patience is key here.
To serve, carefully flip onto the serving plate. Whip cream with powdered sugar, and vanilla. Fold in mascarpone. Top pavlova with whipped cream, then sliced strawberries.: The act of flipping the cooled meringue onto a plate is a quiet, triumphant moment; do it gently so the shell does not shatter. Whipping the heavy cream with powdered sugar and vanilla will make a billowy, slightly sweet topping; folding in Mascarpone cream adds silkiness. When you spread the filling, the contrast between the dark shell and white cream is striking. Finally, arrange the sugared strawberries so they glisten. Avoid overwhipping the filling, which can turn grainy, and place the fruit on just before serving to keep it fresh.