Preheat oven to 350°F and grease the baking tin
Combine brown sugar, cinnamon, butter, and walnuts in a small bowl to create crumble.
Mix wet ingredients – oil, milk, eggs, vanilla – in a medium mixing bowl
Mix dry ingredients – all purpose flour, sugar, salt, baking powder – in a large mixing bowl
Pour wet ingredients into dry ingredients and whisk until just combined. The batter should be quite sticky. Fold in chocolate and walnuts with a spatula.
Pour batter into greased baking tin. Cover the top evenly with crumble.
Bake for 50 - 55 minutes until a toothpick inserted in the center comes out clean. Let cool for 15 minutes before slicing and serving.