In a large bowl, with an electric mixer on medium speed, beat butter and sugar until smooth. Add whole egg and almond extract and beat until well blended. Stir or beat in flour and baking powder until well blended. Cover bowl and chill until dough is firm, about 1 hour.
Shape dough into 1-inch balls, flatten each slightly, and place each about 1 inch apart on buttered or cooking parchment-lined 12- by 15-inch baking sheets.
In a small bowl, beat egg yolk with milk to blend. Brush cookies lightly with egg mixture; discard any remaining. Press an almond into the center of each cookie and sprinkle each with about 1/8 teaspoon sesame seeds.
Bake cookies in a 325° oven until lightly browned, 15 to 20 minutes; if baking more than one pan at a time, switch pan positions halfway through baking. Let cookies cool on sheets for 5 minutes, then use a wide spatula to transfer to racks to cool completely.
Note: Nutritional analysis is per cookie.
You can store the cookies airtight for up to 3 days.
This recipe is very easy to make. Unlike the other reviewer, I did not increase the almond extract...and wished that I had. The cookies were not very almond-y to me, although they were to my husband. I would make again if I needed a specialty cookie.