To make the crumb topping, mix brown sugar, flour, cinnamon. Add oil and combine. Refrigerate while prepping the cake batter.
Preheat oven to 350F and grease your ramekin.
In a small bowl blend oil, milk, vanilla. Add flour, baking powder, pinch of salt and whisk to combine (don't overmix). The batter should be pour-able, if not, add more milk 1 tsp at a time until it is.
Pour cake batter into ramekin and top with the crumb topping.
Bake 20 minutes and enjoy!
Notes
If you want a very sweet cake, use 1 tbsp sugar for the cake batter. This recipe is totally "dessert sweet" as written though.
If you want a quick icing, combine 1 tbsp powdered sugar with 1 tsp maple syrup, drizzle!