Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large mixing bowl, whisk together the 3 large eggs, 1 teaspoon of pure vanilla extract, 1 teaspoon of almond extract, and the zest of 1 lemon until well combined.
Melt the ½ cup of butter and allow it to cool slightly before adding it to the egg mixture. Stir until fully incorporated.
In another bowl, whisk together the 2 cups of all-purpose flour, 1 cup of granulated sugar, 1 teaspoon of baking powder, and ½ teaspoon of salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix.
Gently fold in the whole almonds and sliced almonds until evenly distributed in the dough.
Divide the dough into two equal portions. On the prepared baking sheet, shape each portion into a log about 12 inches long and 3 inches wide. Flatten the tops slightly.
Bake in the preheated oven for 25-30 minutes, or until the tops are golden and firm to the touch.
Remove the logs from the oven and let them cool for about 10 minutes. Once cool enough to handle, slice each log diagonally into ½-inch thick pieces.
Arrange the biscotti slices cut-side down on the baking sheet. Bake for an additional 10-15 minutes, flipping halfway through, until crisp and golden.
Remove from the oven and let the biscotti cool completely on a wire rack before serving.
