Lightly oil a 20cm square cake tin, then line with baking paper, allowing the paper to overhang the sides by a few centimetres (this will make it easier to lift the slice from the tin).
Pulse Hass avocado, dates, nuts and coconut oil in a food processor until smooth, scraping down the sides if necessary, until incorporated. Add chia seeds, cacao powder and coconut, pulse to combine. Transfer to a bowl, add mixed seeds, cacao nibs and ½ tsp sea salt, mix well then press evenly into prepared tin. Smooth the top, scatter with a little extra coconut and press into the surface. Refrigerate overnight until firm.
Use the paper to lift the slice from the tin, then cut into bars and refrigerate in an airtight container for up to 2 weeks.