Beat the butter until it has a light and fluffy texture.
Add the sugar, continuing to beat, until thoroughly combined.
Add the egg and 1 tablespoon of the milk, and beat until well blended.
In a second bowl, add the flour, baking powder, and salt. Toss with a fork until blended.
Add the candied ginger and orange zest to the dry ingredients, reserving about 1 tablespoon of the ginger and 1 teaspoon of the orange zest.
Add the dry ingredients to the wet. You should end up with a thick cookie dough, and you may have to knead it by hand.
Divide the dough into three even sections, and form each into a 7-inch cylinder.
Mix the coarse sugar with the reserved candied ginger and orange zest and sprinkle it on your counter. Roll the dough cylinders in it – you shouldn’t use it all up just yet. Wrap the dough cylinders in wax paper or plastic wrap, and refrigerate for about 2 1/2 hours. The dough should be firm enough ti slice.
When it’s ready, preheat the oven to 350 F.
Cut the rolls into cookie slices about 1/4-inch thick. Arrange them on an ungreased baking sheet, about 1-inch apart. Brush the tops with the 2 tablespoons of milk, and sprinkle with the remaining sugar-orange zest-ginger mixture.
Bake for about 8 minutes, until the edges are golden brown.