Step 1 Stir together the flour, sugar, baking powder, salt and coarsely chopped hazelnuts in a large bowl. Using a pastry blender, a mixer with the flat beater attachment or your fingertips, work in the butter until the mixture resembles coarse meal. Mix in the cream and hazelnut liqueur to form a soft dough.
Step 2 Knead for just 10 seconds, then lightly flour the dough and pat it out on a baking sheet in a 1-inch-thick layer. Cover with plastic wrap or wax paper and refrigerate for for 1 to 8 hours.
Step 3Heat the oven to 375 degrees, placing a rack in the middle position. Uncover the dough, and use a round cutter to cut out 2 1/4-inch biscuits. Pat the scraps together and re-cut to make a total of 8 biscuits. Arrange them at 2-inch intervals on a nonstick or parchment-lined baking sheet.
Step 4Brush a light coating of egg wash on the biscuits. Sprinkle 1 teaspoon of the finely chopped hazelnuts and one-half teaspoon sugar on each biscuit.
Step 5Bake for 20 minutes, or until slightly tanned and springy. Transfer the baking sheet to a rack to cool.