Preheat the oven to 350ºF. In a medium bowl, whisk together the melted butter, brown sugar, eggs, molasses, and vanilla until smooth.
Add the flour, spices, baking powder, and salt. Use a rubber spatula to lightly mix the dry ingredients together on top, then fold the dry into the wet to create a homogenous dough. Allow the dough to sit and hydrate for 10 minutes. It will be very sticky.
Line a large baking sheet with parchment paper. Lightly flour your hands and divide the dough in half. Place the dough on the parchment paper and use your hands to shape 2 logs, approximately 10 x 3.5-inches and ¾ inch thick. Dust the logs with powdered sugar.
Bake for 30 minutes, until dry to the touch. Remove the baking sheet from the oven and allow the logs to cool for 15 minutes. They can crumble if you cut them when they are too hot, and also if too cool.
Use a sharp serrated knife to cut each biscotti log into slices about ⅝ inch wide, parallel to the short edge. For longer biscotti, you can slice diagonally. You should yield approximately 25-30 biscotti total.
Place the biscotti slices back on the parchment-lined baking sheet, cut side down. Bake again at 350ºF for 10 minutes, then flip the biscotti onto the other side and bake for an additional 10 minutes. They will be crisp and slightly golden. The biscotti will dry out more as they cool.
Allow the biscotti to cool on the baking sheet until you are able to handle them, then transfer to a wire rack to cool completely, about 2 hours.
Chop the white chocolate bar and melt over a double boiler or in the microwave. Stir every 20 seconds until melted. Add 1 teaspoon of oil to thin the chocolate, if needed.
Place the wire rack of biscotti over a piece of parchment paper to catch the drips. Use a spoon or piping bag to drizzle the melted chocolate over the biscotti. Let sit for 30 minutes so the chocolate can solidify.
Biscotti can be stored in an airtight container at room temperature for 1-2 weeks.