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.