Preheat oven to 350°F (175° C). Grease or line an 8” x 8” (20cm x 20cm) baking pan with parchment.
Make the streusel:
In a small bowl combine the sugar, oats, flour, and cinnamon. Stir to distribute everything evenly.
Pour in the melted oil and water. Use a fork to stir vigorously until everything is wet and forms crumbles.
Make the swirl:
Combine the sugar, cinnamon, and coconut oil
Make the cake:
In a medium bowl combine the flour, baking powder, baking soda, and salt. Stir to evenly distribute everything. Having the dry ingredients combined before adding to the wet ingredients reduces the amount of stirring needed which keeps a light and delicate cake.
In a large bowl, combine the sugar and oil. Whisk to blend.
Add in the oat milk, vinegar, and vanilla extract. Whisk until smooth and well emulsified.
Add in the flour mixture. Stir until just smooth. Do not over mix.
Pour half the batter into prepared baking pan and shake to level, pushing all the way to the corners as needed.
Spread the cinnamon swirl paste over the surface of the batter. Pour the remaining batter in an even layer. Sprinkle the streusel over the top of the batter.
Bake for 35-40 minutes until a skewer inserted comes out clean. Allow to cool in the pan for at least 10 minutes before removing and allow to cool completely.
Store at room temperature for up to 3 days or in the freezer for up to 1 month. Cake improves when left to sit for a day before eating.
Notes
Mix the wet and dry ingredients separately: Mixing the dry ingredients together before adding in the wet ingredients yields a softer cake, as there’s less chance of overmixing.
Level the batter: When pouring the initial batter, shake the baking pan a bit to make sure that it reaches each and every corner.
Toothpick test: The coffee cake is done baking when a toothpick inserted into the pan comes out clean.