Preheat the oven to 325ºF.
Line an 8x8 square pan with parchment paper and set aside.
In a large bowl, combine the melted butter and brown sugar, stirring until well combined.
Mix in the oats, milk, and vanilla extract. Stir until well combined.
Press the oat mixture into the 8x8 baking pan using your hands or the back of a metal spoon until pressed in firmly.
Bake for 22-25 minutes.
Let cool completely before cutting into 10 bars.
Store in an air-tight container for 5-7 days.