Whisk together the ground almonds, flour, and salt in a bowl. Set aside. Beat the butter, and sugars until well mixed. Add in egg, almond extract and beat until blended. Sift in the flour mixture and stir until mixed.
Wrap the dough with plastic film and chill for at least 2 hours or overnight. Prepare 2 large baking trays and line with baking paper. Remove the dough and roll teaspoons of dough into a finger shape. Place a blanched almond half into one end of the cookie to form a nail and squeeze in the center of the dough create a knuckle shape. Use a small knife to cut a few small ridges in the knuckles. Place them back to the fridge to chill 15 minutes.
Preheat the oven to 190C/375F. Remove the cookies from the fridge and bake for about 10 minutes or until lightly golden brown. Remove cookies from the oven and allow them to cool for 5 minutes. Lift up the almond half, put a little bit of the strawberry jam around the edges of the nail base, and press the almond half back into place, so the jam oozes out from around the edges. Place them on a wire rack to cool.