In a medium bowl, mix together brown sugar, all-purpose flour, salt, and ground cinnamon. Cut in cold butter until mixture resembles coarse crumbs. Set aside.

Preheat your oven to 325°F (163°C) and line an 8x8-inch baking pan with parchment paper.

In a large mixing bowl, whisk together flour, sugar, baking powder, baking soda, and salt.

In a separate bowl, whisk together sour cream, eggs, vanilla extract, and milk until smooth.

Add softened butter and oil to dry mixture; mix until it resembles wet sand. Then incorporate wet ingredients until just combined.

Spread half of the batter into the pan, sprinkle half of the streusel topping, layer remaining batter, and top with the rest of the streusel.

Bake for 65-75 minutes until a toothpick comes out clean or with moist crumbs.

Cool in the pan on a wire rack before slicing into squares.