For the almond meringue, set the oven rack in the middle position and preheat the oven to 425°F (220°C). Line a 12 x 17-inch (30 x43‑cm) sheet pan with parchment paper and set aside.
In a small bowl, combine the ground almonds, 2 tbsp (30 mL) sugar, and the cornstarch.
Place the egg whites in the bowl of an electric mixer fitted with the whisk attachment. Begin mixing the egg whites on medium speed until foamy. Increase the speed to medium-high and continue beating to soft peaks. Add the remaining 1 cup (250 mL) sugar in a slow stream.
Once all the sugar is added, stop the machine and scrape down the sides of the bowl using a rubber spatula. Continue beating until the whites form stiff shiny peaks. Add the vinegar and almond extract and mix just to combine.
Remove the bowl from the mixer and gently but thoroughly fold the ground-almond mixture into the meringue, one-third at a time.
Spread the meringue in the pan, levelling the top. Sprinkle with sliced almonds and very lightly press them in with your hand.
Place in the oven, and immediately reduce the temperature to 275°F (140°C). Bake until the meringue is set and light golden brown, about 40 minutes. Cool on a wire rack for 10 minutes.
Sift icing sugar over a clean sheet of parchment paper and turn the meringue out onto the paper, almond-side down. Carefully remove the parchment liner.