Begin by preheating the oven to 350°F/175°C. Next, ensure you've greased and floured an 8-inch/20cm square cake pan.
To prepare the crumb topping, combine sugars, cinnamon, and salt in a medium bowl. Use a fork to blend in the melted butter, and then add the flour, stirring until it's just moist. Set this mixture aside.
Begin by blending the dry ingredients (flour, baking soda, baking powder, salt, and cinnamon) in a large bowl. In a separate medium-sized bowl, whisk together yogurt, brown sugar, oil, and maple syrup.
Begin by finely chopping one apple and folding it into the wet ingredients. Then, take the other apple and slice it thinly. Toss these slices in lemon juice and set them aside.
Combine dry with wet ingredients, being careful not to overmix. The resulting batter will have a thick consistency. Pour it into the prepared pan and ensure a smooth surface.
Begin by lining the cake's surface with apple slices, then generously spread the crumb topping over them.
Bake for 50 minutes, or until a toothpick emerges clean. Allow it to cool for a minimum of 10 minutes before slicing and serving, whether you prefer it warm or at room temperature.