Mix flour, salt and baking powder together, set aside. Beat sugar and softened butter together until creamy.
Add half the flour, and then add eggs, vanilla and lemon peel. Gradually add the rest of the flour until a somewhat stiff batter is achieved. Spread batter evenly into a parchment lined, round, 10” spring form pan.
Slice cherries and prune plums in half and remove pit. Push fruit into cake batter as closely as possible to each other.
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To make the streusel topping – With fingertips rub together butter, sugar and flour until crumbly spread over top of cake.
Bake at 350°F for 45 minutes