Preheat your oven to 350F. Lightly oil or coat an 8-inch / 20cm square baking dish with oil or vegan butter.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, and salt.
In another medium mixing bowl or large Pyrex liquid measuring cup, combine the sour cream, non-dairy milk, olive oil, cane sugar, and vanilla. Mix well, then pour these wet ingredients into the dry ingredients. Fold the batter together until it's fully mixed (some small lumps are fine).
Pour half of the batter into your prepared baking dish. Use a small, inverted spatula or the back of a spoon to smooth this mixture evenly over the bottom of the cake pan. Sprinkle the cinnamon sugar evenly over this layer.
Pour the remaining batter on top. Use a regular or an invert spatula or spoon to smooth this layer over. It’ll be a little tricky to do this—be patient and confident!
Sprinkle all of the streusel topping over the top layer of cake batter.
Transfer the coffee cake to the oven. Bake for 40 minutes, or until the top is puffy, the streusel is a deep golden color, and a toothpick inserted into the cake emerges clean. Transfer the cake to a cooling rack and allow it to cool in its baking pan.
Cool the cake for 2 hours before slicing and enjoying.