Preheat the oven to 350 degrees and line a baking sheet with parchment paper.
In a mixing bowl, cream the softened butter and sugar until light and fluffy.
Add the egg yolk and vanilla extract, then mix until combined.
Stir in the flour, cocoa powder, and salt until a soft dough forms.
Turn the dough out onto the prepared sheet and press it into a flat rectangle about 1/2 inch thick.
Bake for 20 minutes until the edges are set.
Remove from the oven, let cool slightly, then slice the rectangle into 8 wing-shaped pieces.
Return the slices to the oven for 10 more minutes to crisp.
Melt the dark chocolate chips in a small bowl.
Dip one end of each cooled biscotti into the melted chocolate to create the wing effect, then set on parchment to harden.