How to Make It
Preheat oven to 225°F. Whisk together cornstarch and 1 1/4 cups of the sugar. Beat egg whites with an electric stand mixer on medium-high speed 1 minute; add cream of tartar and salt, beating until blended. Very slowly add sugar mixture, 1 tablespoon at a time, beating until mixture is glossy, stiff peaks form, and sugar has dissolved. Add vanilla, beating until incorporated.
Spoon mixture into 8 tall mounds on 2 parchment paper-lined baking sheets. Flatten mounds slightly into 4-inch rounds. Using the back of a spoon, make an indentation in center of each to hold filling. Bake in preheated oven until very pale and golden, about 2 hours. Turn oven off; let meringues stand in oven, with door closed and oven light on, 12 hours.
Stir together strawberries, elderflower liqueur, mint, and remaining 2 tablespoons of the sugar. Let stand, stirring occasionally, until sugar is dissolved, about 15 minutes. Stir together mascarpone and cream of coconut until smooth. Beat heavy cream on high speed until stiff peaks form, 2 to 3 minutes. Fold whipped cream into mascarpone mixture.
Spoon mascarpone cream into center of meringues; top with strawberries and accumulated juices. Sprinkle with coconut.