Mix all the ingredients in a small bowl until well combined. Set aside.

Add the brown sugar, flour, cinnamon, and salt to a medium bowl. Whisk together then add the vegan butter. 

Use your clean fingers to press and crumble the butter into the dry mixture. Once you get a crumbly mixture, set aside. 

Preheat the oven to 350ºF or 180ºC, and lightly grease a 9x13 inch cake pan (about 23×33 cm). Set aside.

Mix the plant milk and vinegar in a jar or bowl until well combined. Let the mixture rest at room temperature for at least 5 minutes. After this time, the milk will thicken and curdle.

Add the milk and vinegar mixture and beat again until combined.

Incorporate all the remaining ingredients (flour, baking powder, and salt) and mix until well combined. 

Pour half of the batter into the prepared cake pan.

Sprinkle the cinnamon layer over the batter in the pan, then pour the rest of the cake batter on top. 

Finally, top with the streusel and bake for about 50-55 minutes or until a toothpick inserted in the center comes out clean. Baking time may vary depending on the oven.

Allow it to cool in the pan for 20 minutes before cutting.

Serve your vegan coffee cake immediately or store the leftovers in an airtight container in the fridge for 3 to 5 days. You can also freeze it for up to 1 month. Transfer from the freezer to the fridge one day before decorating/serving.