Warm the syrup or treacle, butter and milk together until all is melted. Leave to cool. When the mixture is almost cold, add the dark brown sugar, spices and two thirds of the flour sifted with the baking powder. Then mix to a smooth dough with the rest of the flour. If it is very sticky, leave the dough in the refrigerator for a while. Roll out to about 6mm, ¼” thickness and cut into rectangles of 25mm by 70mm, 1”×3”.

Place on a greased tray and cover with hazelnut halves stuck on with egg yolk. Either use a small paint brush or your index finger to put a small dab of yolk on the flat side of the halved nut.

Bake at about 180°C, 350°F. for 10 to 15 minutes. They don't spread very much, even on silicon release paper.

When cold, coat with the icing mixture.

Makes 70 - 80 biscuits, 4 trays full.

Notes:
Hazelnuts have very thin skins, it is not practical to blanch and de-skin them in the usual manner. The skin often comes off when they're cooked - maybe you could try a light roasting then a rub between your hands, before halving the nuts? I have heard of bakers rubbing a handfull of the nuts in a tea towel.

A soak in boiling water softens the nuts sufficiently to halve them easily. But you still have to look for the cleavage plane in each nut. And then dry the halves again, afterwards!

Icing is made by sifting the icing sugar and cocoa together, mix with the hot water and then stir in the Copha. Don't have the icing too thick.

Ref. Dr. Oetker p.167