Preheat the oven to 350°F (175°C). Grease a 9x9-inch baking pan or line it with parchment paper.
In a large bowl, combine the flour, oats, brown sugar, baking soda, and salt. Stir in the melted butter until crumbly.
Press half of the mixture into the bottom of the prepared baking pan to form the crust.
In a separate bowl, mix the sweetened condensed milk with the chopped Butterfinger candy bars. Pour this mixture over the crust.
Sprinkle the remaining crumb mixture over the Butterfinger layer, pressing it down gently.
Bake for 25-30 minutes, or until the top is golden brown. Let it cool in the pan for about 10 minutes before transferring to a wire rack.
Once cooled, melt the chocolate chips in a microwave-safe bowl in 30-second intervals, stirring until smooth. Drizzle over the cooled bars.
Allow the chocolate to set before cutting into squares.