Preheat oven to 350°F (180°C). Line an 8" cake round with parchment paper (no need to line the sides) and lightly grease with butter. Set pan aside.
In a large measuring cup, pour in the milk and 1 tbsp apple cider vinegar. Set aside to thicken and curdle for 3-5 minutes. (don't panic if any lumps form, this is normal)
Meanwhile, grab a large bowl and whisk together the flour, sugar, baking powder, cinnamon and salt. Pour in the milk mixture and oil, stirring until a thick batter has formed. If using grated or chopped apples, mix them in now.
Transfer the batter to the pan making sure to smooth the top with a spatula or spoon. If using sliced apples like I do, add the slices right on top of the batter.
Take the streusel out of the fridge and use your hands to break the dough into large crumbs. Tossing them right on top of the batter (or in my case apples). Slide the cake pan into the oven and bake for 50-60 minutes or until a skewer inserted at the center comes out clean.
Leave the cake to cool for 5-10 minutes then unmold to cool completely on a wire rack. Serve cake warm with a dusting of powdered sugar. Enjoy!