These spice-laden cookies originated in colonial New England; the candied ginger is our own addition. As the name seems to imply, the bars are best after being hidden away for a day or two, when the flavors have had a chance to deepen.
Preheat oven to 350 degrees. Butter a 10-by-15-inch baking pan, and line the bottom with parchment paper. Butter the parchment, and set pan aside. Combine flour, baking powder, baking soda, ginger, cinnamon, nutmeg, cloves, salt, and pepper in a medium bowl; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, cream butter on medium speed until smooth. Add sugar; beat until light and fluffy. Beat in egg, yolk, and molasses.
Add flour mixture; beat on low speed until combined. Add 1/2 cup candied ginger and the raisins; beat until combined.
Spread dough into the prepared pan in an even layer, and bake until firm to the touch, 18 to 22 minutes, rotating pan halfway through. Remove from oven; let cool completely in pan before icing.
Drizzle with icing, and then sprinkle with remaining 1/2 cup candied ginger. Let stand until icing has set, about 15 minutes. Serve cut into squares.