Preheat the oven to 180° C.
In a large, glass bowl, melt the butter in the microwave until melted.
Set aside to cool for a couple of minutes.
Add the peanut butter, vanilla, brown sugar and egg, and mix well using either a whisk or a spatula.
Fold in the flour with a wooden spoon or spatula, and mix just until you can't see the flour anymore.
Fold in the chocolate chips.
Spoon the batter into a baking tray with high edges that you've oiled or buttered beforehean. Smooth out the batter so that it's approximately the same thickness everywhere.
Bake for aproximately 20 minutes, but be sure not to overbake them because then they can get quite dry.
Cut into bars or squares and remove them from the pan once they've cooled down a bit.