Step 1Adjust the oven rack to the middle position and heat the oven to 325 degrees. Spread the walnuts on a baking sheet and toast in the oven until lightly browned, about 6 to 8 minutes, shaking the pan halfway through to ensure that the nuts toast evenly. Remove from oven and cool completely. Increase the oven heat to 350 degrees.
Step 2In a food processor fitted with the steel blade, combine 1 cup of the nuts with 3 tablespoons sugar and process until the mixture is the consistency of fine meal. Add 3 more tablespoons sugar with the flour, baking powder and salt and pulse to incorporate. Add the butter and pulse on and off until the mixture is the consistency of fine meal.
Step 3Transfer the mixture to a large bowl and stir in the chocolate pieces. Make a large well in the center and pour in 1 cup whipping cream, the creme fraiche and the vanilla. Whisk the liquids together. Using one hand, draw in the dry ingredients and mix until just combined. The mixture will be crumbly.
Step 4Wash and dry your hands and dust them with flour. Turn the dough out onto a lightly floured work surface and knead gently a few times, then gather it into a ball. Roll or pat the dough into a circle 1 1/2 inches thick. Spray the inside of a 1 1/2 -inch round cutter with nonstick spray and cut out circles, cutting them as close together as possible and keeping the trimmings intact.
Step 5Gather the scraps, press them back together, and cut out additional circles. (If the dough gets too soft to cut, refrigerate it for 15 minutes.) Place the circles in groups of three on a parchment-lined baking sheet, spacing the groups 1 inch apart and gently pressing the edges together to form clovers. Freeze until firm, about 1 hour.
Step 6Brush the tops with the remaining 2 tablespoons cream and sprinkle with the remaining 1 tablespoon sugar. Using the large hole of a box grater, grate some of the remaining one-fourth cup nuts over the top of each scone. Bake for 30 to 34 minutes, until slightly firm to the touch and lightly browned.