Whisk together flour, baking powder, baking soda and salt in a medium bowl. Set aside.
In the bowl of a stand mixer, fitted with the paddle attachment, combine butter, sugar and eggs. Beat for 3 - 4 minutes, until light and fluffy.
Add lemon juice, lemon extract and citrus zests. Mix to combine.
Add dry ingredients and mix until combined.
Divide the dough in half. On a parchment-lined baking sheet, form each half into a rectangle of dough approximately an inch thick. **Dough will be slightly sticky. Use damp (not wet!) hands to form the dough into rectangles.
Cover with plastic wrap and refrigerate for 20 - 30 minutes.
While dough is chilling, preheat oven to 350°F / 325°F with convection fan / 177°C / Gas Mark 4.
Bake for 30 minutes, rotating pan halfway through. Remove from oven and cool for 10 minutes.
Gently slice biscotti into 1/4" pieces and lay them back on the sheet pan on their sides. Bake for an additional 10 minutes for crispy biscotti. *See notes for extra crunchy cookies.
Cool on wire cooling rack completely before drizzling with chocolate if desired.
To drizzle with chocolate: Melt 1/3 cup chocolate in a microwave safe bowl in 30 second increments, stirring between each. When there are pea-sized pieces of chocolate left, you're done. Let the chocolate sit, the heat will continue melting it. Stir until smooth, then transfer melted chocolate into pastry bag or zip-top bag with a corner snipped off to pipe melted chocolate on top of cookies.