In a medium bowl, cream the butter and sugar. Add egg and almond essence; mix until fluffy. Stir in flour, baking powder and salt; mix well.
Divide dough into 4 pieces, and roll each one into a log about 30cm long. Place 2 logs per cookie sheet 10-13cm apart. Flatten each roll by hand until it is about 3 inches wide. Brush flattened roll with milk and sprinkle with sliced almonds.
Bake in preheated oven 12 to 15 minutes or until edges are slightly browned. While the cookies are still warm, cut them crosswise at a diagonal, into slices about 2cm wide. When cool, drizzle with almond icing if you like.
Almond icing: In a small bowl, stir together powdered sugar, almond essence, and milk until smooth. Drizzle over the cookies.