1 1/4 cups pure maple syrup (amber or Grade B)
3/4 cup heavy cream
2 teaspoons cider vinegar
Pinch of salt
6 tablespoons unsalted butter, softened
1/3 cup sugar
1 large egg
1/2 teaspoon vanilla
1 cup cake flour
1 teaspoon baking powder
1/4 teaspoon salt
Put oven rack in upper third of oven and preheat oven to 350°F.
Stir together maple syrup, heavy cream, cider vinegar, and pinch of salt in a small saucepan and bring to a boil, then remove from heat.
Beat together butter and sugar in a bowl with an electric mixer until light and fluffy, about 1 minute.
Add egg and vanilla, then beat until just combined (batter will be very thick).
Sift flour, baking powder, and salt together into egg mixture and stir with a rubber spatula until just combined.
Pour 1/3 cup syrup mixture into 8-inch glass baking dish.
Divide batter in bowl into 6 mounds with rubber spatula and spoon each mound onto syrup mixture in baking dish, spacing mounds evenly.
Pour remaining syrup mixture over and around mounds.
Bake until topping is golden and firm to the touch, 25 to 30 minutes.