For the meringue:
4 large egg whites at room temperature
¼ c. dark brown sugar
¾ c. superfine sugar
Pinch of salt
½ tsp. vanilla
2 tbsp. unsweetened cocoa powder
Dark chocolate cream:
4 large egg yolks
¼ c. sugar
½ c. plus tbsp. heavy cream
½ c. whole milk
¼ tsp. coarse salt
8 oz. dark chocolate (70% cacao), finely chopped
Milk chocolate curls:
4 oz. milk chocolate, coarsely chopped
Make the meringue: Preheat oven to 300º; line a rimmed baking sheet with parchment paper. Draw an 8-inch circle on the parchment, then flip. Mix egg whites, sugars, and salt in a mixing bowl set over a pan of simmering water; whisk constantly until sugars dissolve and mixture is warm, about 3 minutes. Remove from heat, and whisk on medium high speed until stiff peaks form, about 8 minutes; beat in vanilla. Sift cocoa powder over meringue and fold until barely any streaks remain. Spread meringue onto the parchment, using the circle as a guide (meringue will spread during baking). Bake until dry to the touch, about 1 hour; let cool on wire rack.
Make the dark chocolate cream: Prepare an ice water bath. Whisk egg yolks and 2 tbsp. sugar in a medium bowl. Bring cream, milk, salt and remaining sugar to a simmer in a medium saucepan. Pour a third of the hot cream mixture into the yolk mixture, whisking until combined. Pour yolk mixture into pan with hot cream. Cook over medium heat, stirring constantly, until the mixture coats the back of a spoon, about 6-7 minutes. Place the chocolate in a large heatproof bowl. Pour the hot mixture over the chocolate and whisk until chocolate melts and mixture is smooth. Strain through a sieve into a medium bowl, then set in an ice-water bath, stirring often, until cold.
Make milk chocolate curls: Melt chocolate in a heatproof bowl set over a pan of barely simmering water, stirring occasionally. Spread evenly onto an upside-down rimmed baking sheet; refrigerate until set, about 15 minutes. Using a sturdy metal spatula, scrape chocolate at a 45-degree angle to form curls.
To assemble pavlova: Spread dark chocolate cream evenly in the center of the meringue, leaving a ½ inch border from the edge. Spread whipped cream over chocolate cream, if desired. Garnish with chocolate curls and serve immediately.