Arrange racks in lower and upper thirds of oven; preheat to 350°. Line 2 rimless baking sheets with parchment paper. Pulse hazelnuts and oats in a food processor until coarase meal forms. Set aside.
Melt butter in a medium saucepan over medium heat. Add both sugars and honey; whisk until blended and sugar dissolves, 1–2 minutes. Remove from heat. Add nut mixture, flour, and salt; stir until well blended.
Spoon batter by 1.5-teaspoon portions onto baking sheets, spacing 2 1/2" apart. Using your fingertips, pat cookies down to 1/4"-high rounds; push in any jagged edges to form smooth circles. Each cookie sheet should only have about 8-10 cookies on it (these cookies spread out a lot while baking!!)
Bake, rotating sheets after 6 minutes, until dark golden brown and cookies spread out into a thin layer, 10–12 minutes. Slide cookies carefully onto a wire rack and let cool.
Using a pastry brush, brush half of each cookie with melted chocolate. Let stand until the chocolate is set, about 2 hours. DO AHEAD: Can be made 2 days ahead. Store airtight in between sheets of parchment or waxed paper.
