Preheat your oven to 350°F (175°C) and grease and line with baking paper two 8-inch round pans.
Add the butter and mix on low speed until mixture resembles like wet sand.
Combine the wet ingredients, zest and eggs in a jug and mix with a fork just to break down the eggs.
Add about three quarters of the wet ingredients in the bowl and beat on medium to medium-high speed for 2 minutes, scraping the bowl half way. You will see the mixture to lighten in color and get fluffier.
Add the rest of the wet ingredients and beat for another 1 minute until well combined.
Divide the mixture in two pans and bake for 40-50 minutes or until a toothpick inserted in the center comes out clean.
Invert onto a rack and let the cakes cool.
Add the salt, cinnamon, vanilla and two tablespoons milk and beat to incorporate.
When the cakes have cooled, frost with buttercream and decorate with hazelnuts and inverted rosemary sprigs.
All of the ingredients should be at room temperature (about 71 Fahrenheit / 22 Celsius)
If your home is colder than that, open the oven light (only the light, NOT the oven) and place the ingredients inside for about half an hour.