Preheat the oven to 180°C (350°F) and spray a 22cm /9in ring tin with cake release.
Sift the flour, baking powder and salt into a bowl. Mix the orange zest with the sour cream.
Put the butter and sugar in the bowl of your stand mixer fitted with the paddle attachment. Beat for several minutes, scraping the bowl midway, until fluffy and light.
Gradually add the eggs, adding 1 tbsp of flour to avoid curdling.
Alternate adding the flour and sour cream, beating briefly on medium-low speed between each addition. Scrape the bowl as needed to make sure everything is well blended.
Transfer the batter into the prepared tin, levelling it with a palette knife.
Bake for 45-50 minutes or until the cake is golden, springy and coming away from the edges of the tin.
Cool in the tin for 10 minutes then carefully turn out onto a cooling rack.
To make the candied oranges, put all the ingredients in a saucepan and heat until the sugar dissolves. Add the orange slices and simmer for 10 minutes, or until the rind is softened. Place the slices on a cooling rack set over a tray (to catch any drips) and leave them to cool.
Mix the icing sugar with enough orange juice to make a thick but pourable glaze.
Spoon the glaze over the cooled cake, letting it drip prettily down the sides.
Decorate the cake with the candied blood orange slices if you like.
Reserve any syrup from making the candied orange slices to brush over cakes or use in cocktails.Instead of blood oranges, you can use any type of citrus.