Melt butter and cinnamon over medium heat. Stir in apples, sugar & 3 tablespoons water.
Cover and cook stirring occasionally for 4-6 minutes or until very slightly softened.
In a small dish combine cornstarch and water. Add to pan while stirring and continue to cook until apples are soft (not mushy) and filling is thickened. Let bubble 1 minute. Cool.
Some apples are juicier than others. If you would like your filling thicker, combine 1 tablespoon cornstarch with 1 tablespoon water. Add a little at a time while the mixture is boiling until you reach desired consistency.