In a small bowl combine the raisins, vinegar and 1 cup of water. Let stand at least 10 minutes. Strain the liquid into a bowl and reserve the raisins for garnish.
Add the pine nuts, the butter and the sugar to a medium saucepan over low heat. Stir until the sugar is melted but not caramelized, about 4 to 5 minutes. Slowly pour in half of the vinegar and water mixture, then the flour, stirring constantly, until the mixture becomes dense. Discard the remaining vinegar and water mixture. Add the prosciutto, sprinkle with a little bit of black pepper and remove from the heat.
Coat the bread slices with oil and arrange them on a sheet pan. Bake in the oven until lightly toasted, about 3 minutes. Spread the prosciutto mixture on the crostini and arrange on a serving platter. Garnish with the reserved raisins and serve.
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