Preheat the oven to 340°F.
Line three 6-inch round cake tins.
Mix the milk and vinegar, then set aside to thicken.
Add the remaining wet ingredients to the milk mixture.
Sift in the dry ingredients and mix until combined.
Pour the batter into the tins.
Bake for 20-30 minutes, or until cooked through.
Let the cakes cool before leveling them off.
Make the cinnamon sugar glaze by combining all the ingredients.
Once the cakes have cooled, pour the glaze onto the sponge and let it soak in for 10 minutes.
Using a hand mixer or paddle attachment on your stand mixer, cream the butter on medium-high until creamy and light in color.
With the mixer on low, add the powdered sugar bit by bit, scraping down the sides of the bowl after each addition. Add the cinnamon and milk, then mix until combined.
Assemble the cake by layering the sponge with the cinnamon frosting.