Preheat the oven to 350 degrees. Line a 8 X 8 inch pan (or 9 X 9 inch) pan with aluminum foil so that it hangs over the sides. Apply non-stick spray.
Whisk together the flour, baking powder, cinnamon, cream of tartar and salt in a medium size bowl and set aside.
In a large bowl, mix the brown sugar and butter. Add the eggs and vanilla and mix until well combined. Add the dry ingredients. Add the apple pieces. Spread the batter in the prepared pan.
Whip out a squeeze bottle or jar of dulce de leche or caramel. Drizzle over the batter.
Combine the strudel ingredients in another bowl and chop together until the butter is mostly down to pea-size chunks. Sprinkle evenly over the top of the batter.
Bake for 40-45 minutes or until the top is lightly golden and a toothpick inserted into the center of the pan comes out clean. Allow to cool completely. Lift the bars out by the overhanging foil and cut into bars. The edges will be crisper and more solid, while the center may be messier and sticky with caramel. It's all good.