Sift together the flour, baking soda, baking powder, salt, nutmeg, and cinnamon in a bowl.

In a separate bowl, cream together the softened butter and brown sugar until light and fluffy using a mixer.

Add the eggs, vanilla, pumpkin puree, and orange zest to the butter mixture and beat until well blended.

Gradually add the flour mixture to the wet ingredients and beat at low speed until just incorporated. Be careful not to overmix.

Stir in the raisins, if using.

Cover the dough and refrigerate for about 1 hour to make it easier to handle.

Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

Divide the chilled dough in half.

On a lightly floured surface, shape each half into a log about 10 inches long.

Place the logs a few inches apart on the prepared baking sheet.

Bake for about 30 minutes, or until the logs are golden brown and firm to the touch.

Remove the logs from the oven and let them cool on a wire rack for 10 to 15 minutes.

Transfer the logs to a cutting board and slice each log on the diagonal into 12 slices.

Place the biscotti slices cut-side down on the wire rack, which is placed on the baking sheet.

Bake for another 15 to 20 minutes, or until the biscotti are golden brown and crisp, turning once if needed to prevent burning.

Remove the biscotti from the oven and let them cool completely on the wire rack.

Melt the white chocolate in a double boiler or microwave, adding a small amount of shortening if using chocolate chips to help with smoothness.

Drizzle the melted white chocolate over the cooled biscotti.

Refrigerate briefly to set the chocolate.