Pre-heat the oven to 190°C (375℉), and prepare a muffin tin (I used cupcake cases).
Melt the butter and set to one side to cool a little.
Whisk the sugars and egg together until light and fluffy.
Add the butter, oil, milk and vanilla and whisk until everything is combined.
Sift the flour, cinnamon and baking powder together and stir carefully into the wet ingredients until just combined.
Using a spoon, dollop the batter into the muffin cases until almost full, leaving a small space (see note 2).
Using your hands or a spoon, sprinkle over the streusel topping.
Bake for around 18 minutes. The cakes are ready when an inserted skewer comes out clean. Serve warm. Enjoy!