Combine the flour, salt, baking powder, and 1 teaspoon cinnamon in a bowl. In a large mixing bowl, beat butter with the sugar until light and fluffy with electric mixer. Beat in eggs and vanilla extract. Add the flour mixture to the butter/sugar mixture, alternating with the milk abd beating just until blended. Carefully spoon batter evenly over the apple slices (batter will be thick); gently spreading to cover. Bake for 55 minutes, or until a toothpick inserted in the center of the cake comes out clean.