Grind the walnuts. Combine flour, sugar and ground walnuts in a bowl. Add the butter in slices and the amaretto. Chop everything util crumbly, then knead until smooth.
Divide the dough into walnut-sized balls (about 15 grams, approximately 1/2 ounce) and place on a parchment paper-lined baking sheet, keeping some distance between each. Gently press a walnut half onto each ball of dough. Bake in a preheated oven at 175°C (gas mark 2-3, fan: 160°C) (approximately 350°F) for about 15 minutes. Remove from oven, sprinkle with powdered sugar and serve.