• Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius) and line an 8x8 or 9x9 inch square pan with parchment paper, making sure there is overhang on at least two sides. This will make it easier to remove the coffee cake from the pan. Set aside.

Brown Sugar Cinnamon Filling

• 
  Whisk together the brown sugar, flour, and cinnamon. Set aside.

Streusel

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  Whisk together the flour, brown sugar and cinnamon. Cut the cold and cubed butter into the mixture using a pastry cutter, two forks, or your hands, until it is completed incorporated. The mixture will resemble wet sand. Set aside.

Cake

• Whisk together the flour, baking powder and salt in a medium-sized bowl. Set aside.
• 
  In a medium-sized bowl or in the bowl of a stand mixer fitted with the paddle attachment, beat together the room temperature butter and vegetable oil until they are completely homoegenous. This won't happen unless the butter is completely at room temperature. About 2-3 minutes.
• 
  With the mixer on low, stream in the granulated sugar then beat on medium speed for 1-2 minutes until light and fluffy.
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  With the mixer on low, add the eggs one at a time, mixing well and scraping down the sides and bottom of the bowl in between each addition. Add the vanilla extract and mix until combined.
• 
  Add half of the dry ingredients and mix on low until just combined. Add half of the milk and mix on low until just combined. Repeat this process with the other halves of the dry ingredients and the milk. Do NOT overmix.
• 
  Pour half of the batter into your prepared pan and spread into an even layer.
• 
  Sprinkle the filling mixture on top in an even layer. It is going to seem like a lot of filling.
• 
  Pour the other half of the batter on top and spread into an even layer. The filling might make it a little difficult to spread this layer of filling on top, but just work at it with a rubber/offset spatula.
• 
  Top with the streusel, sprinkling it on so that the top is evenly covered.
• Bake for 35-45 minutes until the edges are set and slightly pulling away from the pan. A toothpick entered into the center should pull out clean or with a few moist crumbs. Let cool slightly, top with powdered sugar, slice and enjoy!