Preheat oven to 325º F. Lightly butter 10 4¼-inch tart pans and set them on a large baking sheet.
For the topping, whisk together flour, brown sugar, cinnamon, and salt in a large bowl. Stir in melted butter and mix until combined and crumbly. Break up any large clumps with your fingers as needed.
To prepare the cake batter, whisk together flour, sugar, baking powder, and salt in a small bowl.
In a separate bowl, combine the egg, milk, oil, and vanilla, whisking until well blended.
Add the flour mixture to the egg mixture and stir until just combined. Do not over mix.
Using a 1/4 cup ice cream scoop add about 3 tablespoons of batter to each tart pan. Spread the batter to the pans' edges with the back of a small spoon. It will look quite thin.
Using a 2 tablespoon ice cream scoop, transfer the crumb topping to the cakes, dividing it between the cakes. Spread the topping evenly to the pans' edges and gently press it into the batter.
Bake until the cakes rise and a tester inserted in the center comes out clean, about 25 to 28 minutes,
Remove the crumb cakes from the oven and cool for 5 to 10 minutes before unmolding them. Serve warm.