Preheat the oven to 400 F/205 C. Wash, peel, core, and slice the apples into small chunks.
In a large bowl whisk together the egg, buttermilk, sugar, and melted butter. Melt the butter in the microwave or stovetop, until barely melted and not hot.
Sift together the flour, baking soda, baking powder, salt, and cinnamon and add it to the bowl of wet ingredients. You can whisk them together if you don't have a sifter.
Mix the dry ingredient in with the wet using a whisk. Then stir in the chopped apples. The batter is ready to go. Pour the batter into an 8x8 inch baking pan, or use a 10-inch round or even a springform pan that has been greased with shortening and floured or sprayed with cooking spray. Spread it out evenly with a spatula.
In a small bowl mix together softened butter, flour, sugar, and cinnamon with a fork until it is crumbly and resembles small chunks of damp sand.
Sprinkle the streusel topping evenly over the cake and bake it in the oven, middle rack position, for 25 to 30 minutes until a toothpick comes out clean when poked in the center. Once it comes out of the oven allow it to cool for 25 to 30 minutes.
Serve the cake with a scoop of vanilla ice cream, milk, etc. Enjoy!