Preheat the oven to 180°C/160°C fan and line two baking trays with baking parchment.
Melt chocolate in a microwave safe bowl in a microwave in 20 second bursts (stirring in between) until just melted then set to one side.
Beat the egg whites on high in a stand mixer until satiny peaks form. Gradually add in the sugar a spoonful at a time and beat on high until the meringue mixture is glossy and stands in stiff peaks.
Using a spatula quickly fold in the lemon juice and vanilla extract, then fold in the corn starch, cocoa powder and melted chocolate until just blended.
Using a large star nozzle, pipe your meringue mixture in small circles on your parchment paper, leaving a little room between each to allow for spreading in the oven.
Turn your oven down to 130°C/110°C fan and bake for 1 hour and 15 minutes, then turn the oven off and without opening the door, let the meringues sit in the oven for another 30 minutes.
Transfer the meringues with the parchment paper onto the kitchen side or a cooling rack and allow them to cool to room temperature.
To prepare the strawberries:
Toss together the strawberries and sugar and let sit for at least 15 minutes until juices develop.
Set half the strawberries to one side and take a fork and mash the remaining strawberries in a bowl.
To make the whipped cream:
In the bowl of a stand mixer fitted with a whisk attachment, beat the double cream until thickened. Add the sugar slowly while continuing to whisk, followed by the vanilla. Beat into soft peaks.
To make the Eton Mess:
Crush the cooled meringues (setting a few whole meringues to one side, enough to garnish each bowl) and fold into the whipped cream. Spoon the mashed strawberry mixture into the bottom of your serving bowls/glasses. Spoon the cream and meringue mixture on top and garnish with sliced strawberries and a chocolate meringue, sprinkling over any extra crushed meringue.
Note: This dessert is best served within a few hours of assembly. If preparing far in advance, store the meringues, whipped cream, and strawberries separately and only combine just before serving.