• Preheat oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper.

Make the Streusel

• In a medium bowl, whisk together the flour, brown sugar, granulated sugar, cinnamon, and salt.
• Add the cold butter and cut it in using a pastry cutter or your fingertips until pea sized crumbs form.
• Add the vanilla and gently toss together. Refrigerate while you make the batter.

Make the Cake

• In a large bowl, beat the butter, sugar, and oil together on medium speed for 2–3 minutes, until pale and fluffy.
• Add the eggs one at a time, mixing well after each addition.Mix in the vanilla. Scrape down the bowl as needed.
• In a separate bowl, whisk together the all-purpose flour, cake flour, cinnamon, baking powder, baking soda, and salt.
• Add the dry ingredients to the butter mixture in three additions, alternating with the sour cream and milk. Mix just until combined. The batter should be thick, fluffy, and spreadable.

Assemble

• Spread half of the batter evenly into the prepared pan. Remove the refrigerated streusel from the fridge and sprinkle about ⅓ of it evenly over the batter. Squeeze some of the streusel together with your hands to create larger bakery-style crumbs.
• Top with the remaining batter and gently spread it out. Finish with the remaining streusel on top, again squeezing together to create some larger crumbs. It will be a thick layer.
• Bake on the middle oven rack for 45 to 55 minutes, or until the top is golden brown and a knife inserted into the center comes out clean.
• Allow the cake to cool for at least 30 minutes before slicing and top with a dusting of powder sugar. Enjoy!