Combine the heavy cream and saffron in a small saucepan. Bring to a bare simmer over medium heat, then remove from the heat and transfer to a bowl; let stand, stirring often, 10 minutes. Transfer to the refrigerator and let cool, stirring occasionally, about 20 minutes; stir in the vanilla. Combine the raisins and sherry in a small bowl and microwave 45 seconds; set aside.
Preheat the oven to 400 degrees F and line a baking sheet with parchment paper. Whisk the flour, granulated sugar, baking powder and salt in a large bowl. Work in the butter with your fingers until the mixture looks like coarse meal. Add the raisin mixture, then add the saffron cream, reserving 1 tablespoon (with a few of the threads) for brushing. Mix with a wooden spoon to make a soft dough.
Turn the dough out onto the prepared baking sheet and form into a 7-inch round, about 3/4 inch thick. Cut into 8 wedges and pull them about 1 inch apart. Brush with the reserved saffron cream and sprinkle with the coarse sugar.
Bake until the scones are puffed and golden brown around the edges, 14 to 16 minutes. Transfer to a rack and let cool slightly. Serve with butter.