Reserve 12 of the pecan halves. Put the remaining nuts in a food processor and whizz until the consistency of ground almonds. Add the egg yolks, 100 g (3½ oz) of the sugar, the breadcrumbs and melted butter and blend until smooth. Turn into a bowl.
Whisk the egg whites in a large clean bowl with a hand-held electric whisk until peaking. Gradually whisk in the remaining sugar, a spoonful at a time. Stir a third of the mixture into the pecan mixture using a large metal spoon. Add the remaining egg whites and stir gently until just combined.
Divide the mixture evenly between paper cupcake cases arranged in a 12-hole cupcake tray. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 18–20 minutes until just firm to the touch. Leave to cool for 10 minutes, then transfer to a wire rack to cool completely.
Make the buttercream. Beat together the butter, sugar and vanilla until pale and creamy. Add the measurement water and beat again until paler in colour. Spread over the cupcakes and decorate each with a pecan half.