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.
Notes
You can use any kind of milk or milk substitute
If you want to make muffins, use a 12 inch. muffin tray and bake for 25 - 30 minutes.