Preheat oven to 175°C. Line a loaf pan with parchment paper.
Mix all cinnamon filling ingredients into a paste.
Whisk together oil, both sugars, eggs and vanilla. Stir in sour cream and buttermilk. In a separate bowl, mix the dry ingredients, then gently fold into the wet ingredients. Fold in the carrots. Do not overmix.
Pour half the batter into the pan. Add half the cinnamon filling and swirl gently. Repeat with the remaining batter and filling.
Bake for 50–60 minutes, or until a toothpick comes out with a few moist crumbs. Cool completely.
For the walnuts, heat honey over low heat, add walnuts and stir until lightly caramelized. Add salt and cool.
For the frosting, brown the butter until golden and nutty. Cool slightly, then beat with cream cheese and powdered sugar until smooth.
Spread over the cooled cake, top with caramelized walnuts, slice and enjoy
