Preheat oven to 350°F Fahrenheit. Butter and flour a 9-inch square metal baking pan, knocking out excess flour.

Make Brownie Layer : whisk together flour, baking powder, and salt in a small bowl. Melt chocolate and butter in 2-quart heavy saucepan over low heat, stirring until smooth, then remove from heat. Cool to lukewarm, then stir in brown sugar and vanilla. Add eggs 1 at a time, beaten with a wooden spoon after each addition until mixture is glossy and smooth. Add flour mixture and stir until just combined. Spread batter evenly in baking pan and bake in middle of oven until a tester comes out clean, 30 to 35 minutes. Cool completely.

For Caramel Layer: bring sugar, corn syrup, water, and a pinch of salt to a boil in a 3-quart heavy saucepan over moderate heat, stirring until sugar is dissolved, then boil, without stirring, until mixture turns a golden caramel color, about 10 minutes. Remove from heat and carefully add cream and vanilla. Cool completely.