Use a small saucepan to melt the cream cheese, butter, and almond butter together on low heat, stirring frequently. (If you prefer to use a microwave, combine them in a microwave-safe bowl, then heat for 20 seconds, adding an additional 10 seconds if needed.)
Once those ingredients are melted, scrape the mixture into a bowl, adding the protein powder, erythritol, and vanilla extract. With a hand blender or immersion blender, blend until smooth; this usually takes 30 seconds or slightly longer. It's very important that you don't skip this step, though, as it's key to developing the right consistency: Smooth, fudge-like, slightly sticky.
Scrape the now smooth mixture into a six-inch square baking dish lined with parchment paper, then place in the refrigerator for two hours or longer so the fudge can harden appropriately. (Freezing sometimes causes it to harden too quickly, and can upset the texture.)
With a sharp knife, cut into 36 squares, keeping refrigerated in an airtight container until ready to serve.