Preheat oven to 350 degrees. Lightly grease a 12-cup muffin tin or use paper liners. For muffins, cream butter in mixer bowl; gradually add sugar and continue to beat until light and creamy. Add eggs and beat another 1-2 minutes. Add vanilla, cinnamon,and nutmeg. Stir together flour, baking powder, and salt. Add flour mixture alternately with milk, beginning and ending with flour. Lightly fold one of the diced apples into the batter. Spoon muffin mixture into prepared muffin tins, filling about 3/4 full. Place remaining diced apple on top of each muffin. In a small bowl, mix together streusel topping ingredients until coarse crumbs are formed. Top each muffin with streusel topping and bake for 25-30 minutes, or until a toothpick inserted in center comes out clean. Let muffins cool on a wire rack for 15-20 minutes. Heat caramel sauce to warm it slightly. Drizzle over tops of muffins using a piping bag or with a spoon, using more warmed caramel if needed.