Preheat the oven to 375 degrees F. Butter a large sheet pan.
Into a large bowl, sift together the sifted flour, sugar, baking powder, soda, and salt. Transfer 1/2 cup of the mixture to a food processor. Add 1/3 cup of the toasted almonds to the food processor and pulse until fine and powdery, about 45 seconds. Return this mixture to the dry ingredients, stir in the whole almonds, and mix.
In a small bowl, whisk the eggs, Grand Marnier, zest, vanilla, and almond extract. Stir into the dry ingredients. The dough may seem dry, but it will moisten as you mix. Mix until smooth. Turn the dough out onto a lightly floured work surface and knead it a few times just to bring the dough together. Divide into 2 equal pieces. Wet your hands and use them to shape each piece of dough into a flattened log about 8 inches long by 3 inches wide and 1 inch high.
Transfer the logs to the prepared pan, leaving at least 3 inches between the logs (they will spread during baking). Bake until light golden brown, 25 to 30 minutes, rotating the pan after 15 minutes to ensure even baking.
Let cool 15 minutes on the pan. Reduce the oven temperature to 300 degrees F. Carefully transfer the logs to a cutting board. Wipe off the pan and butter it again.
Using a serrated knife, cut the logs into 1/2-inch-thick slices, discarding or snacking on the ends. Arrange the slices on the sheet pan cut-side up. Bake until toasted, 20 to 25 minutes. Turn the oven off, prop the oven door open, and let cool completely in the oven.
Meanwhile, in the top of a double boiler (or in a bowl) set over hot (not boiling or simmering) water; slowly melt the white chocolate, stirring often.
Line a sheet pan with waxed or parchment paper and transfer the melted white chocolate to a small bowl. Dip 2 inches of the end of each cookie into the white chocolate and turn chocolate-side-up so that the chocolate drips unevenly down the cookie. The chocolate-covered end of the biscotti is its head; the uneven edge is the bottom edge of the ghost's "sheet." Lay them on the waxed paper as they are dipped.
While the chocolate is still warm, decorate the chocolate covered ends with little ghost faces, using small candies, licorice, candy corn, chocolate chips, raisins, or sprinkles. If necessary, snip the candies to make them smaller. Sprinkle orange, yellow, or black sugar on the top edges of the biscotti for hair or all over the ghost's face. Let cool on wire racks until set. Store in an airtight container.