Cream together butter & sugar in a large mixing bowl. Add eggs & vanilla. Whisk together flour, soda, salt, oatmeal and add to sugar mixture. Mix till crumbly and set aside.
Heat chocolate chips, sweetened condensed milk, and butter in a glass bowl in the microwave, stirring every 30 seconds until smooth. Stir in vanilla.
In bottom of greased 9x13" pan spread 2/3 of the crumb mixture. Spread filling over crust, then drop spoonfuls of remaining mixture on top of the chocolate.