Preheat oven to 375°F. Line a large baking sheet with a silpat or parchment paper.
Beat the almond paste, almonds and sugar together on low speed - it will be crumbly.
Beat in the egg white, almond and vanilla extract on medium speed until a smooth dough forms.
Divide the dough into 12 equal pieces (use a kitchen scale if you have one) and roll into a ball.
Working with one ball at a time roll it under your palm to form a 4+1/2-inch long log.
Place it on the baking sheet and shape it into a horseshoe.
Beat the egg with a tablespoon of water in a small bowl. Brush the cookies with it, one at a time and sprinkle the top with almonds pressing them on to stick.
Bake 12 - 14 minutes until golden on the bottom and edges. Cool on the pan.
Add ¾ of the chocolate to a microwave proof bowl. If you have a chocolate setting on your microwave use that to melt the chocolate (use the normal setting if you don't) in 20 second increments, stir after each until completely melted and hot.
Stir in the butter and remaining chocolate, stir, stir, stir until completely melted.
Dip the ends of the cookies into the chocolate and place on a wire rack or parchment paper to harden. If the chocolate remains tacky to the touch refrigerate for 20 minutes to help it set up.
Store tightly covered in a cool place up to a week.