Heat the oven to 350° and coat a baking sheet with cooking spray. In a medium bowl, whisk together the flour, baking powder, spices, and salt. In a stand mixer set on medium speed, beat together the butter and sugar until fluffy. Add the egg, molasses, and vanilla extract and mix until well incorporated. Reduce the mixer's speed to low and blend in the flour mixture.
With slightly wet hands, halve the dough and shape each piece into a 2 1/2-by-9-inch rectangle. Transfer the loaves to the prepared baking sheet and bake them until they're golden brown and their tops are just starting to crack, about 25 minutes. Let the loaves cool on the sheet and reduce the oven's temperature to 325°.
Use a sharp knife to cut each loaf into 3/4 inch thick slices. Place the slices cut side down on the baking sheets and bake them until they're toasted, about 8 minutes. The pieces should be crunchy on the outside but not hard. Transfer them to a rack to cool completely.
Melt the white chocolate chips in a heat-safe bowl according to the package directions, then spoon it into a ziplock bag and snip off a corner. Drizzle the biscotti with the melted chocolate. Let the coating set before serving.