Spoon batter evenly into 2 greased and floured aluminum foil-lined 9- x 9- x 2-inch square pans. Bake at 350° for 40 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on a wire rack.
Bring brown sugar, whipping cream, and butter to a boil in a 3-quart saucepan over medium heat, stirring often; boil 1 minute. Remove from heat; whisk in powdered sugar and vanilla until smooth. Add pecans, stirring gently 3 to 5 minutes or until mixture begins to cool and slightly thicken. Pour immediately over cakes in pans. Let cool completely. Cut cakes into squares.