Prep the pan: Line an 8x8-inch baking dish with parchment, leaving a little overhang for easy lifting.
Warm the wet ingredients: In a medium bowl, stir peanut butter, honey, milk, and vanilla until smooth.
If stiff, microwave the bowl for 15–20 seconds to loosen.
Add the dry mix: Stir in oats, ground flaxseed, salt, coconut (if using), and chocolate chips or raisins.
Adjust texture: The mixture should be thick and slightly sticky. If too dry, add 1 tablespoon milk. If too wet, sprinkle in more oats.
Press into the pan: Transfer mixture to the lined dish.
Press firmly with a spatula or the back of a measuring cup to compact evenly into the corners.
Chill to set: Refrigerate for at least 1 hour, or 20 minutes in the freezer for a quick set.
Slice: Lift out using the parchment and cut into 12 bars or 16 small squares for younger kids.
Serve: Enjoy as-is, or drizzle with a tiny bit of melted chocolate for a treat.