Add the hazelnut flour, cake flour, and baking powder to a bowl and whisk to combine. Set aside.
In a large bowl, beat eggs, salt, and sugar with a hand mixer until it has at least doubled in size, about 3 minutes. Add the melted butter and espresso powder. Stir to combine.
Add the dry ingredients and gently fold until just about combined. Add the slivered almonds and mini chocolate chips and stir until combined. Cover the bowl and chill until cold, about 1- 2 hours.
About 30 minutes before you are ready to take the dough out of the fridge, preheat the oven to 350°F and line a baking sheet with a silicone mat or parchment paper.
Turn the cookie dough out onto the prepared baking sheet and shape into a 12 inch log. I suggest wearing food safe gloves as the dough may be quite sticky. It also helps to use two cake scrapers to shape the dough.
Bake the log for 25- 30 minutes, or until light golden brown. Cool on the baking sheet for 15 minutes before carefully transferring to a cooling rack.
Cool completely, at least an hour or two. When the log is nearly cool, preheat oven to 325°F. Slice into 1 inch cookies on a slight diagonal. Place the cookies upright on the lined baking sheet. Insulate the bottom of the baking sheet by placing it inside a second baking sheet of the same size. Bake for 40 minutes, rotating the pan halfway through. Place cookies on a cooling rack and cool completely.
Once the cookies are cool, chop your chocolate as finely as possible and place in a microwave safe bowl. Microwave on HALF power for 30 seconds, stir, then 20 seconds, stir, then 10 seconds, stir. The chocolate will still be quite chunky, but keep stirring and it should become smooth. If it doesn't, microwave on HALF power for 10 second intervals, stirring well between each.
Pour the melted chocolate onto a warm plate large enough to fit a long biscotti cookie. You want the chocolate in a rough rectangle the size of a biscotti. Gently dip a cookie into the chocolate and wiggle around to get chocolate up the sides a bit. Let any extra drip off. Place the cookie on a piece of wax paper and continue with the remaining cookies. Let the chocolate set, then serve or store in an air tight container.