How to Make It
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.
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.
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.
Unsweetened whipped heavy cream or crème fraîche.