Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat.
In a large mixing bowl, beat 3 eggs with vanilla extract, almond extract, lemon zest and melted butter. Mix well to combine.
In a separate bowl, whisk together the flour, sugar, baking powder, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until a soft dough forms. Fold in the whole and sliced almonds, ensuring they're evenly distributed throughout the dough.
Divide the dough into three equal portions. On a lightly floured surface, shape each portion into a log approximately 8–10 inches long and 2 inches wide. Place the logs on the prepared baking sheet, spacing them at least 2 inches apart.
In a small bowl, whisk together the remaining egg. Brush the tops and sides of each log with the egg wash. For extra crunch, you can sprinkle additional sliced almonds on top.
Bake the logs in the preheated oven for 40 minutes, or until they are golden brown and firm to the touch. Remove from the oven and let them cool on the baking sheet for about 10 minutes.
Reduce the oven temperature to 325°F (160°C). Transfer the slightly cooled logs to a cutting board. Using a sharp serrated knife, slice each log diagonally into ½-inch-thick slices.
Place the slices cut-side down back onto the baking sheet. Bake for 10 minutes, flipping the biscotti halfway through the baking time to ensure even crispness. They should be lightly golden and dry to the touch.
Remove the biscotti from the oven and transfer them to a wire rack to cool completely. Once cooled, they're ready to enjoy!