Preheat oven to 375°F (190°C). Lightly spray a 1½-quart baking dish with cooking spray; use a pastry brush to spread spray evenly, removing excess.
For sauce: In a medium saucepan, bring cream, maple syrup, and salt to a boil over medium-high heat, stirring frequently; cook, stirring frequently, for 5 minutes. Remove from heat, and let cool for at least 5 minutes.
For cake: In a medium bowl, whisk together flour, baking powder, and salt.
In a large bowl, whisk together melted butter, sugar, eggs, and vanilla; whisk in milk. Gradually add flour mixture, whisking until just combined and no dry streaks remain. Spread batter into prepared pan. Slowly and gently pour sauce onto the back of a large spoon over batter so it runs off spoon and onto batter. (Do not stir together.)
Bake until cake is golden, sauce is bubbly, and a wooden pick inserted in center comes out clean, 25 to 30 minutes. Let cool for 5 minutes. Serve warm with Whipped Crème Fraîche. Cover and refrigerate for up to 2 days.