Place chocolate in a heatproof bowl and microwave in 30-second increments, stirring between each, until chocolate is melted. Add tahini and whisk until combined and smooth. Stir in nuts. Spread into an 8x8-inch pan that's been lined with parchment paper. Refrigerate until set, at least 3 hours, then cut into 36 squares. Keep in the fridge as they will start to melt at room temperature.