Preheat your oven to 350°F (175°C). Grease your baking pan with a bit of melted butter or cooking spray to prevent sticking.
In a large mixing bowl, combine the melted butter, maple syrup (or brown sugar), apple butter, and vanilla extract. Whisk together until smooth. Add in the eggs, one at a time, mixing well after each addition.
In another bowl, whisk together the whole wheat pastry flour, baking soda, cinnamon, and kosher salt. Gradually add the dry mixture to the wet ingredients, stirring gently until just combined.
Fold in the semi-sweet chocolate chips with a spatula, ensuring they are evenly distributed throughout the batter.
In a separate bowl, mix the all-purpose flour, packed brown sugar, cinnamon, and melted butter for the crumb topping. Stir until the mixture resembles coarse crumbs.
Pour half of the batter into the prepared pan and spread it evenly. Sprinkle half of the crumb topping over the batter. Next, pour the remaining batter on top and finish with the rest of the crumb topping.
Place the pan in the preheated oven and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. The top should be golden brown and slightly set.
Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes. Transfer the cake to a cooling rack to cool completely before slicing.
