Preheat oven to 180C (350F).

Grease and line 2 loaf pans with baking paper.

Place butter and sugar in food processor and whizz until light and fluffy and sugar has dissolved.

With motor still running add eggs, one at a time, until well combined and vanilla paste.

Add milk, sour cream & flour and whizz until combined (but not too long).

Combine extra caster sugar and cinnamon in a small bowl.

Pour 1/4 of batter into each pan, sprinkle with a generous layer of the cinnamon sugar, leaving enough for topping the cakes, then top with remaining batter.

Insert a knife into the batter and gently swirl it to create swirls of cinnamon.

Sprinkle the tops with the remaining cinnamon sugar.

Bake for 45-50 minutes, or until an inserted skewer comes out clean.