Mix the angel food cake mix with the crushed pineapple in a mixing bowl with a large spoon. Mix just fold in well blended. DO NOT use an electric mixer. That will beat too much air into this.
Pour the batter into a 9"x13" baking pan that has been sprayed well with nonstick spray. Bake at 350 degrees for 25 to 30 minutes. Remove from oven and cool. Before cutting in squares, run a sharp knife around the edges of the pan.
To make the pineapple sauce, melt the butter in a deep sauce pan on medium heat. Add the brown sugar and stir until it has dissolved and slightly caramelized. Add the pineapple and salt. Mix the cornstarch with the 1/4 cup cold water until smooth. Stir into the pineapple sauce and bring up to a boil for a couple of minutes. Don't let it boil over and stir frequently. Remove from heat and stir the vanilla in. Serve while warm over cake slices. This also makes a great topping for vanilla ice cream. But the cake is delicious without topping also!