Heat oven to 350 degrees F. Spray 9 x 13 x 2-inch cake pan with baking spray containing flour.
In a mixer, beat butter and sugar until well blended, light and fluffy. Beat in eggs one by one. Mix in sour cream and vanilla.
In a separate bowl, combine flour, salt, baking powder, baking soda, and cinnamon. Add to wet ingredients in mixer, combining well. Stir in apples. Spoon batter into the prepared baking pan. Bake 40-45 minutes. Let cool on cooling rack for a few minutes, then spread prepared frosting (below) over top.
In a large saucepan over medium heat, melt and combine butter, evaporated milk and sugar. Reduce heat to low and let cook for 1 1/2 - 2 hours until mixture has thickened and turned a golden caramel color. Remove from heat. Stir in vanilla. Spread over warm cake. Let cool completely before serving.