In a small microwave safe bowl add chocolate chips and heat in 20 seconds increments, stirring after every increment until melted and smooth. Set aside.
Add egg whites, sugar, salt and cream of tartar or lemon juice in a heatproof large mixing bowl.
Set bowl over a saucepan filled with 1 inch simmering water. The bowl shouldn't touch the water.
Heat the egg whites stirring frequently until the sugar has dissolved and the mixture feels warm to the touch, about 3-4 minutes.
Beat the egg whites with an electric mixer on medium-high speed until tripled in volume, stiff peaks form and you can turn the paddle upside down without the meringue falling.
Add vanilla extract and whisk to combine.
Pour the slightly cooled but still liquid melted chocolate into one side of the meringue and using a large spatula gently swirl it in 3-4 big folds to leave large streaks of chocolate.
Gently drop dollops of meringue over the chocolate cake batter and spread it across the cake surface, making big swoops into the surface of the cake to bring small amounts of the batter to the top, beginning to marble the meringue. The meringue will have big swirled peaks.