In a large bowl, whisk together the melted butter, brown sugar, maple syrup, and vanilla until smooth.
Add the egg and whisk again.
Stir in the flour, rolled oats, and a pinch of salt (if using) until combined. The mixture should be sticky.
Fold in the mini chocolate chips. Reserve a few for sprinkling on top.
Line an 8x8 inch (20x20 cm) baking pan with parchment paper.Press the mixture firmly and evenly into the pan — this helps the bars hold together once baked.
Bake at 350°F (180°C) for 22–25 minutes, or until the edges are golden and starting to come away from the edge of the pan. For softer bars, bake slightly less; for firmer bars, bake a minute or two longer.
Let the bars cool completely in the pan, then slice into rectangles or squares.Tip: Chill for 20 minutes before cutting for clean edges.