Start by preparing the garnish; boil the zest three times in a saucepan, using fresh water each time in order to remove any bitterness. Strain the zest and reserve to one side.
Combine the sugar with 500ml of water in a saucepan and bring to a simmer, stirring gently to dissolve the sugar. Simmer for 4-5 minutes, then add the boiled zest and simmer for a further 5-6 minutes. Strain the candied zest, reserving the syrup, and arrange them spaced out on inverted sieves to dry.
Preheat the oven to 170°C.
Grease and line a 8 inch square baking tin. Combine the butter, sour cream, sugar, flour, eggs and vanilla extract in a large mixing bowl. Beat until smooth using an electric hand-held whisk; 1-2 minutes. Spoon into the prepared loaf tin and bake for 30-40 minutes until a cake tester comes out clean from the centre of the cake.
Remove and allow to cool on a wire rack for 10 minutes before turning out from the tin and allowing to cool further. When ready to serve, top with the candied orange zests and spoon over some of the orange syrup before slicing and serving with the yogurt.
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