Preheat your oven to 350°F and line an 8x8 inch pan with parchment paper.
Make the crumb topping: stir the flour, brown sugar, cinnamon, and salt together, pour in the melted butter, and mix until big clumps form. Place in the fridge while you make the batter.
In a large bowl, whisk the sour cream, oil, milk, sugar, and vanilla until smooth.
Fold in the flour, baking powder, baking soda, and salt just until no dry streaks remain.
Spread the batter into the pan, then squeeze the chilled crumb mixture into a mix of big and small clumps and scatter over the entire surface.
Bake for 38-45 minutes, until a toothpick inserted in the center comes out clean.
Cool for at least 30 minutes, whisk the powdered sugar and milk into a glaze, drizzle over the top, and slice.
