Place chocolate chips in a microwave-safe bowl (preferably glass). Microwave in 30 second intervals at 50% power, stirring between each interval, until smooth. Set aside.
In a separate bowl, scoop the insides from the avocado and mash with a fork. (The measurement doesn't need to be exact, but it should be about 2/3 cup once mashed). Pour most of the melted chocolate (reserving about 3 tablespoons) into the bowl with the avocado and stir well. (The reserved melted chocolate is for drizzling/topping at the end, if desired. Cover it with plastic wrap and leave it on the counter for now).
Add crushed pecans (reserving about 2 tablespoons for topping, if desired), vanilla, and salt to the chocolate avocado mixture and stir until well combined. Cover the bowl with plastic wrap and place in the refrigerator for 30 minutes to chill.
Meanwhile, combine cocoa powder and espresso powder in a small, shallow bowl and stir well.
After chilling, the truffle mixture should be somewhat easy to scoop and roll into balls. (If not, you may need to chill it longer). Use a small cookie scoop or tablespoon to scoop the chocolate avocado mixture into balls about 1 inch in diameter, rolling them in your palms to shape. Place each ball into the cocoa mixture and roll it around until it's well-coated. Then, place it onto a sheet pan covered with wax or parchment paper.
If topping with chocolate drizzle and chopped pecans: reheat the reserved melted chocolate in the microwave until it has a drizzly consistency. (Avoid overheating, as it can scorch). Using a spoon, drizzle over each truffle, then sprinkle the reserved chopped pecans on top.
Place the sheet pan in the refrigerator for 5-10 minutes to set the chocolate drizzle. Then, transfer truffles to an airtight container and store in the refrigerator until ready to eat!