Preheat the oven to 350F and line a large baking sheet with parchment paper. 

Add the softened butter, granulated sugar, and lemon zest to the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until light and fluffy– about 2 minutes. 

Next, add the eggs and lemon extract. Mix to combine. Don’t worry if the batter looks like it has curdled– it will all come together! 

Add flour, almond flour, and baking powder. Mix to combine and scrape down the sides of the bowl as needed. 

Add the sliced almonds and mix very quickly to distribute them– you don’t want to break them up too much. 

Transfer the dough to the parchment lined baking sheet and use a rubber spatula to shape it into a smooth rectangle that is 4” wide, 1” tall, and 15” long. You can also wet your hands a little and use them to shape the dough if it is sticking to the spatula. 

Bake the biscotti for 20 minutes or until a toothpick inserted into the center comes out clean. 

Carefully transfer the biscotti log to a cutting board and use a serrated knife to slice it into 1” pieces. Place the biscotti back onto the baking sheet, making sure to place them at least an inch apart. 

Bake the biscotti for another 20-25 minutes or until they are golden brown and crisp. 

Allow the biscotti to cool completely on the tray before adding the lemon drizzle. 

In a small bowl, whisk together the powdered sugar, lemon juice, and 1 tablespoon of milk. If the drizzle is too dry, add the second tablespoon of milk. 

Spoon the drizzle over the cooled biscotti. Allow the drizzle to set completely before storing.