Step 1

Preheat the oven to 350°. Lightly butter an 8-inch square glass or ceramic baking dish. Line the bottom of the dish with a piece of parchment paper and butter the paper. In a medium saucepan, boil the maple syrup over moderate heat until reduced to 1 cup, about 6 minutes. Remove the syrup from the heat, whisk in 3 tablespoons of the butter, then pour into the prepared baking dish.

Step 2

In a small bowl, whisk the flour with the baking powder, salt and mace. In another small bowl, whisk the whole egg and egg yolk with the buttermilk and vanilla. In a large bowl, beat the remaining stick of butter with the sugar at medium speed until light and fluffy, about 2 minutes. At low speed, beat in the dry ingredients in 2 batches, alternating with the liquid ingredients; beat until the batter is smooth.

Step 3

Evenly dollop heaping tablespoons of the batter on top of the maple syrup. Bake the pudding for 35 minutes, or until the top is nicely browned and a toothpick inserted in the center comes out clean. Let stand for 10 minutes before serving.