Preheat your oven to 300°F. Line a baking sheet with parchment paper or a silicone baking mat.
In a large bowl, toss the sourdough bread cubes with the melted butter and vanilla extract until evenly coated.
Spread the bread cubes in a single layer on the prepared baking sheet. Bake for 15-20 minutes, tossing halfway through, until the cubes are completely crispy and dried out all the way through. They should feel light and crunchy when you break one open. Remove from oven and let cool slightly.
While the bread is toasting, make the chocolate coating. In a medium microwave safe bowl, combine the chocolate chips, peanut butter, and butter. Microwave on high for 1 minute, then stir. Continue microwaving in 30-second intervals, stirring between each, until the mixture is completely smooth and melted.
Stir the vanilla extract and pinch of salt into the melted chocolate mixture.
Place the toasted sourdough cubes in a large bowl. Pour the chocolate peanut butter mixture over the cubes and gently toss with a spatula until every piece is evenly coated with chocolate. Work gently to avoid breaking the bread cubes.
Add the powdered sugar to a gallon-size ziplock bag. Transfer the chocolate-coated bread cubes to the bag, seal it tightly, and shake vigorously until every piece is completely covered in powdered sugar. Make sure to use a sturdy ziplock bag, not a bread bag or thin plastic bag.
Spread the coated puppy chow out on a parchment-lined baking sheet or clean surface and let cool completely at room temperature until the chocolate sets, about 15-30 minutes.
Once set, transfer to a serving bowl or airtight container. Store at room temperature for up to one week.