10-1/2 tablespoons or 7 ounces sweetened condensed milk
3/4 cup semisweet chocolate chips
1 teaspoon vanilla
Preheat the oven to 350°F. Line an 8-inch square pan with parchment paper cut to fit. Cream the butter until soft, then add the brown sugar and baking soda and cream until light and fluffy, scraping down the sides of the bowl occasionally. Add the egg and vanilla and beat until just combined. Using a strong wooden spoon, stir in the flour and rolled oats until just combined. Press two-thirds of the dough mixture in the bottom of the prepared pan.
Combine the butter, condensed milk, and chocolate chips in a small heavy-bottomed saucepan or microwaveable bowl and place over low heat (or microwave on LOW power), stirring occasionally, until the chocolate has melted. Stir to combine and stir in the vanilla. Spread over the dough in the pan using an offset spatula and dot the surface with the remaining one-third of the dough (it's quite stiff, so use two spoons) irregularly in patches.
Bake for 18-23 minutes or until the top is light brown (the filling will not be set at this point). Cool completely then cut into squares.