Sift together flour, baking powder and salt. Stir in chopped thyme.
Cream butter, sugar, zest and ginger until fluffy. Stir in egg and lemon juice. Add dry ingredients and blend until dough comes together.
Divide dough in half and shape into logs. Wrap and chill for at least 2 hours (or freeze).
Preheat oven to 325 F.
Unwrap dough and slice into thin wafers and place on a greased or parchment-lined baking sheet. Brush cookies with milk and bake for 12-15 minutes, just until edges turn golden.