Grease and line a 20cm x 30cm baking tin with greaseproof paper, leaving an overhang on two sides. Heat oven to 190C/170C fan/gas 5
To make the sugar crust, put the rosemary, orange zest, vanilla seeds and two tbsp of the sugar into a mortar and pound until the mix resembles wet sand and smells deeply fragrant. Stir in the remaining 1 tbsp of sugar and set aside.
Put the caster sugar and orange zest in the bowl of an electric mixer fitted with a paddle attachment, then rub with your fingertips until fragrant (this helps draw out the oils in the zest and perfumes the cake more evenly). Add the butter and beat on medium-high until light and creamy
Add the eggs one at a time, beating well, scraping down the sides of the bowl as required, then beat in the vanilla.
In a second bowl, whisk the flour, baking powder, baking soda and salt. Add the dry ingredients and soured cream to the batter alternately in two or three stages, beginning and ending with the flour mixture. Scrape down the bowl, mix briefly until smooth and evenly combined.
Scrape the batter into the lined tin and smooth out the surface. Arrange the apricots on top, cut side up, without overlapping. Don’t press them down into the batter; they will settle naturally while the cake bakes. Sprinkle evenly with the rosemary-orange sugar.
Bake for 40–45 mins, or until a skewer inserted into the centre comes out clean. Remove and leave to cool in the tin for 15 mins, then transfer to a rack. Serve warm or at room temperature with pouring cream.
