In a medium bowl, combine ¾ cup of brown sugar, ⅓ cup of all-purpose flour, a pinch of salt, and 1 tablespoon of ground cinnamon. Incorporate ¼ cup of cold, cubed butter until the 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 1¾ cups of flour, ¾ cup of sugar, 2 teaspoons of baking powder, ½ teaspoon of baking soda, and ½ teaspoon of salt. In a separate bowl, mix ½ cup of softened butter, ¼ cup of oil, 1 cup of sour cream, 2 eggs, 1 teaspoon of vanilla extract, and ¾ cup of whole milk until smooth. Fold wet ingredients into dry until just combined.

Spread half of the batter into the prepared pan, sprinkle streusel over it, add the remaining batter, and gently spread it out.

Sprinkle the remaining streusel evenly over the top layer of batter.

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

Let the cake cool in the pan for about 15-20 minutes, then lift it out using the parchment overhang, place on a wire rack to cool completely before slicing.