Preheat oven to 350ºF. Butter a 7x11 pan, or line with parchment paper.
Whisk first 5 (*) ingredients until well combined.
Remove 1/2 cup (85g) of dry mixture and place into separate smaller bowl. Whisk the baking powder and baking soda into the original bowl of dry mix and set aside.
In the small bowl, add cinnamon and melted butter. Press with a fork and combine thoroughly until it forms crumbles. This will be the topping. Set aside.
Whisk the egg, oil, and buttermilk together until well combined. Add this to the large bowl of dry mix. Stir to combine with a spatula or spoon until no dry ingredients remain. Don't overmix.
Place batter into 7x11 pan. Sprinkle the streusel topping evenly over the coffee cake batter.
Bake at 325ºF for a glass pan, for 25-30 minutes, or until a toothpick inserted in the center comes out c lean. If you're crazy like me, stick a probe thermometer in the center and check that it's between190-200ºF. Cool in the pan and serve.