Make the biscotti: Heat the oven to 350° F. Place the almonds on a cookie sheet and roast until brown and fragrant. Remove from the oven and allow to cool. Set-aside.
Chop the chocolate bars into small bits. Set-aside.
In a medium bowl whisk together the flour, cocoa, baking powder, and salt. Set aside.
Step 3Beat together the oil and sugar until very fluffy and well blended. Add the eggs, one at a time; mixing each one in before adding the next. Add the espresso and vanilla (and optional liqueur, if using). Mix until the espresso is well incorporated.
Gradually stir the flour into the egg mixture, mix until well blended and smooth. Stir in the nuts and reserved chocolate.
Shape the dough into 2 smooth evenly shaped logs (about 15 by 1 ½ inches); arrange on a greased baking sheet and flatten with lightly floured hands to about 3 inches across. Bake 35 minutes; remove to a rack to cool. when the logs are just cool enough to handle; carefully transfer to a cutting board and cut crosswise on a slight diagonal (for best results, use a serrated knife). Lay the slices back on the baking sheets; return to the oven and bake until slices are firm when lightly pressed; about 16 to 20 minutes. Transfer biscotti to a rack to cool completely.
Prepare the coating:Chop the dark chocolate into coarse pieces. In a double boiler set over barely simmering water, melt the chocolate and olive oil together, stirring until smooth. When the cookies are cooled; dip them to the half-way point in the chocolate. Place on wax paper-lined sheets and allow to sit until the chocolate sets up; about 30 minutes.
Variation: Drizzle the biscotti with the chocolate. To do this: melt the chocolate as described. Spoon the melted chocolate into a pastry bag fitted with a large tip. Quickly drizzle long zigzag lines across the biscotti, going back and forth until all the biscotti are done. Allow to sit for 30 minutes, chocolate side up.