Cut ripe avocados in half, remove pits, scoop flesh into a bowl, and mash until smooth.
Gently melt chocolate using a double boiler or microwave, stirring until smooth; add coconut oil if using.
Temper the avocado by mixing a spoonful of melted chocolate into it, then fold in the rest of the chocolate, vanilla, sweetener, and salt until combined.
Refrigerate the mixture for at least 30 minutes to firm up.
Portion out the chilled mixture, roll into balls, then dust with cocoa powder or dip in melted chocolate. Add toppings if desired and refrigerate until set.
Bring to room temperature for about 10 minutes before serving.
Notes
For a glossy finish, dip truffles in tempered chocolate and sprinkle toppings while still soft.