Preheat oven to 350°F. Line the bottom of a 10x15 inch rimmed baking sheet/jelly roll pan with parchment paper, and grease the pan and parchment. Alternately, you can line the pan with foil, leaving a couple inches overhang on two sides (this will allow you to lift the whole tray of bars from the pan). Grease the foil and pan, set aside.

In a medium bowl, whisk flour, baking powder, baking soda, spices, and salt until combined.

In a large bowl, beat butter and brown sugar with a hand mixer or stand mixer. Beat on medium speed for several minutes, until smooth and creamy. Add egg, egg yolk, and molasses, and beat to combine, scraping the sides of the bowl if necessary. With the mixer on low, gradually add in the flour mixture, mixing just until combined. Fold in the raisins.

Spread the dough evenly onto prepared baking sheet. The dough will be thick. Lightly dampen your fingers with water, if needed, to help press the dough evenly into the pan.

Bake for 18-20 minutes, rotating the sheet halfway through the cooking time. Place the baking sheet on a wire rack to cool completely.

Place brown sugar, milk, and butter in a small saucepan. Cook over medium heat, stirring constantly, until the butter has melted and the sugar has dissolved (several minutes). Remove from the heat and whisk in the vanilla extract and powdered sugar. If the icing is too thin, whisk in a little more powdered sugar. If it's too thick, add a little more milk.

Drizzle the bars with icing. Allow the icing to set (at least 15 minutes), then cut.