Preheat oven to 350 degrees F. Line a large rimmed baking sheet with parchment paper.
Melt the butter in the microwave or in a saucepan.
Combine melted butter, brown sugar, salt, baking soda, flour and vanilla. Stir until smooth, then stir in the oats. If using cinnamon and pecans, add them too.
Pour mixture onto a parchment lined rimmed baking sheet and pat into a rectangle that's about 8x9 inches (doesn't have to be exact). Bake for 10 minutes at 350 F. It will be brown and bubbly when you pull it out of the oven.
Allow the oat mixture to cool for about 5 minutes and then carefully trim edges and score into squares. If the mixture crumbles when you cut it, let it cool a little more. Let it cool completely, then separate the squares.