Preheat oven to 225F/107C. Line a large baking sheet with parchment paper or a silicone mat. Clean mixer bowl and whisk attachment with degreasing dish soap and/or vinegar, to clean the bowl of any oil residue.
Place the egg whites into clean mixing bowl and whisk on high speed for a few minutes, until soft peaks form. Combine the sugar and cornstarch, then slowly add to the egg whites. Once all the sugar is added, turn mixer to high speed and whisk for 7 to 9 minutes, until egg whites form stiff peaks and are glossy. Sift the cocoa powder over the meringue, then whisk for about 1 minute, just until incorporated.
Transfer about 2 cups of the meringue into a pastry bag tipped with a star tip. Transfer the remaining meringue into the center of prepared pan. Use a spoon or spatula to form the pavlova - a tall, round disk. Pipe the remaining meringue onto the paper, forming meringue kisses.
Bake the meringue in preheated oven for 2 hours. After 2 hours, turn the oven off but do not open the oven door! Allow the meringue to sit and cool gradually over 2 to 3 hours.
Prepare the lavender cream. In a small bowl, whisk the cream cheese or mascarpone until light and fluffy. Add the lavender oil and few drops of purple food color. Whisk again until creamy. In a separate bowl, whisk together the chilled cream and sugar for a few minutes, until stiff peaks form. Add the cream cheese mixture and whisk again for about a minute, until stiff peaks form.
Assemble the dessert when ready to serve. Spread half of the lavender cream on top of the chocolate pavlova, then add a layer of fresh blackberries. Top with remaining cream and garnish with more blackberries, meringue kisses and mint leaves.