1. Using a small knife, remove the pits from plums without cutting plums entirely in half. Place plums in a medium heatproof bowl. Bring stock syrup and bay leaves to a boil in a small saucepan over medium heat. Immediately pour syrup over plums and let cool completely.

2. Preheat oven to 250 degrees. Line a baking sheet with a nonstick baking mat or parchment paper; set aside.

3. In the bowl of an electric mixer fitted with the whisk attachment, beat egg whites until soft peaks form. With the mixer on low, slowly add superfine sugar; increase speed and continue whisking until firm peaks form. Add confectioners' sugar and beat until meringue is smooth and shiny and stiff peaks form, 6 to 8 minutes.

4. Drain plums and pat dry with paper towels, removing as much syrup as possible. Gently fold plums into meringue mixture.

5. Using two large spoons, scoop one-quarter of the meringue mixture and pass it between both spoons to form a rounded oval shape; transfer to prepared baking sheet. Repeat process with remaining meringue. Transfer to oven and bake for 2 hours.

6. Cool meringues slightly on baking sheet; carefully transfer to a wire rack and let cool completely in a dry place before serving.