Preheat oven to 350°F (175°C). Line a large baking sheet with parchment paper.
In a medium bowl, whisk together flour, cocoa powder, baking powder, and salt.
In a large bowl, whisk eggs and sugar until light and frothy, about 2 minutes.
Dissolve espresso powder in 1 tablespoon hot water, then add to the egg mixture along with vanilla extract. Whisk to combine.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. The dough will be sticky.
Fold in chocolate chips and almonds (if using) until evenly distributed.
Divide the dough in half. With lightly floured hands, shape each portion into a log approximately 14 inches long and 2 inches wide on the prepared baking sheet, spacing them at least 3 inches apart.
Smooth the tops and sides of each log with slightly wet hands to prevent cracking during baking.
Bake for 25 minutes, or until the logs are firm to the touch and slightly cracked on top.
Remove from oven and let cool on the baking sheet for 15-20 minutes. Reduce oven temperature to 300°F (150°C).
Using a serrated knife, cut each log diagonally into ½-inch thick slices. Use a gentle sawing motion to prevent crumbling.
Arrange the slices cut-side down on the baking sheet and bake for 10 minutes.
Flip the biscotti and bake for another 10 minutes, or until they are dry and crisp.
Transfer to a wire rack and cool completely.
For the optional chocolate drizzle: Melt chocolate with oil in a microwave-safe bowl in 30-second intervals, stirring between each, until smooth.
Drizzle or dip cooled biscotti in melted chocolate and place on parchment paper until chocolate sets.
Store in an airtight container at room temperature for up to 3 weeks.