Sift the
flour, baking powder, and salt into a large
bowl. Cut in the butter until it resembles fine
crumbs.
Mix in the
sugar, egg, and vanilla. Divide the dough in
half. Mix the cocoa and milk in a small bowl.
Knead the
cocoa mixture into one half of the dough. Form
the plain dough into a 12 inch (30-cm) log.
Brush with
cold water. Roll out the cocoa dough on a
surface into a 12 x 6-inch (30 x 15-cm)
rectangle.
Place the
plain dough log in the center of the cocoa
rectangle and wrap the cocoa dough around it.
Wrap in plastic wrap and refrigerate for 30
minutes.
Preheat the
oven to 375°F/180°C/gas 4. Butter three cookie
sheets. Slice the dough 1/4 inch (5 mm) thick
and place 2 inches (5 cm) apart on the sheets.
Bake, one
sheet at a time, for
8-10
minutes, or
until lightly browned. Let cool completely.