Thinly slice a baguette; arrange in a single layer on a baking sheet. Brush with softened butter and sprinkle with salt and sugar. Bake at 425 degrees F until lightly golden, about 7 minutes. Let cool slightly.
Toss 1 cup sliced strawberries, 2 tablespoons superfine sugar and 1 tablespoon balsamic vinegar; set aside 30 minutes. Mix 1/2 cup mascarpone cheese, 1/4 cup confectioners' sugar and a dash of vanilla extract. Spread the mascarpone mixture on the toast rounds; top with the berries and juices.