Preheat your oven to 180°C (convection) or 160°C (fan-forced). Line the bottom and sides of a 20 cm round cake tin.
Add all the dry ingredients (flour, sugar, baking powder, cinnamon and salt) to a large mixing bowl. Mix until there are no lumps.
Add the wet ingredients (milk, oil, vinegar and vanilla) and mix until just combined, then pour the batter into your cake tin.
Add all the cinnamon crumble ingredients to a medium bowl. Mix with a fork until just combined. The mixture should have large crumbles with some sandy bits. Don’t overmix otherwise it will turn into a paste.
Sprinkle the crumble on top of your cake, then bake for 50 minutes or until you can insert a toothpick in the middle and it comes out mostly clean (there will be bits of crumble so just make sure there is no wet cake batter).
Allow the cake to cool in the tin for 30 minutes then cool on a wire rack.
To make the icing, add all ingredients to a medium bowl. The icing should have a thick drizzle consistency. When the cake cools, drizzle on the cake.
Enjoy the cake warm or at room temperature. Store in an airtight container in the fridge for up to 3 days.
