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reheat the oven to 170°C (325°F) Gas 3.
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Put the butter and sugar in a freestanding electric mixer with a paddle attachment (or use a handheld electric whisk) and cream until light and fluffy. Add the eggs one at a time, mixing well and scraping any unmixed ingredients from the side of the bowl with a rubber spatula after each addition. Beat in the vanilla extract, flour and baking powder until well mixed. Add the soured cream and mix well until everything is combined and the mixture is light and fluffy.
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Gently stir in the blueberries by hand until evenly dispersed.
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Pour the mixture into the prepared ring mold and smooth over with a palette knife. Bake in the preheated oven for 40 minutes, or until golden brown and the sponge bounces back when touched. Leave the cake to cool slightly in the mold before turning out onto a wire cooling rack to cool completely.
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When the cake is cold, put it on a serving plate, cover the top and sides with the icing, and decorate with more blueberries. Dust with a light sprinkling of icing sugar
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For Making the glaze
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Place all ingredients in a food processor or blender and process until smooth. Add more blueberries or powdered sugar as necessary until desired consistency (thick yet pourable). You can strain through a sieve to make it extra smooth, but that’s optional. Use immediately.