Creating Cinnamon Streusel Fresh Apple Bars is a delightful process! Follow these easy steps to enjoy this amazing dessert:

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan with butter or cooking spray.
2. Prepare Dry Ingredients: In a large mixing bowl, combine the flour, granulated sugar, brown sugar, ground cinnamon, baking powder, baking soda, and salt. Whisk together until well combined.
3. Mix Wet Ingredients: In a separate bowl, combine the melted butter, eggs, and vanilla extract. Whisk until smooth.
4. Combine Ingredients: Gradually pour the wet ingredients into the dry ingredients, stirring until just combined. Do not overmix.
5. Add Apples: Fold in the diced apples until they are evenly distributed throughout the batter.
6. Transfer Mixture: Pour the mixture into the prepared pan and spread evenly.
7. Prepare Streusel Topping: In a small bowl, mix rolled oats, chopped nuts (if using), and additional cinnamon or sugar if desired. Sprinkle this mixture over the batter evenly.
8. Bake: Place the pan in the preheated oven and bake for 40-45 minutes or until the top is golden brown, and a toothpick inserted in the center comes out clean.
9. Cool: Once baked, remove the pan from the oven and let cool in the pan on a wire rack for about 15-20 minutes.
10. Slice and Serve: After cooling, cut into squares or bars and serve warm or at room temperature.