Preheat the oven to 350°F (175°C). Grease the cake pan.

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

In another bowl, mix the buttermilk, vegetable oil, and vanilla extract.

Combine the wet ingredients with the dry ingredients and mix until just combined.

In a separate bowl, combine brown sugar, flour, butter, and cinnamon. Mix until crumbly.

Pour half of the cake batter into the prepared pan. Sprinkle half of the streusel topping over it.

Add the remaining batter and top with the remaining streusel.

Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.

Let cool before serving.