Preheat oven to 165C. Beat butter in an electric mixer until very soft and pale (3-5 minutes), then transfer to a bowl. Clean the mixer bowl, then whisk the yolks until creamy, gradually adding sugar and whisking until yolks are pale and creamy (4-5 minutes). Beat in butter in batches until smooth, then add rind, sift over flour and baking powder, and fold until just combined into a soft dough. Spoon into 10 greased and floured 5cm-6cm ring moulds (or a 23cm ring mould) placed on an oven tray lined with baking paper and bake until puffed, golden and sablés just spring back when touched lightly (15-25 minutes). Set aside to cool, then remove rings and transfer to a platter.
Whisk cream, mascarpone and crème fraîche until soft peaks form. Arrange blood orange slices on top of sablé Breton, spoon on cream mixture and serve scattered with finely grated pistachios and blood orange rind.