Crunchy on top, with an gooey, fudgy chocolate centre. I hope these traybake bars turn out as well for you as they do for me! I place a large piece of foil under the tin as it can sometimes overflow. Well worth the little bit of mess!
Combine 225g butter, brown sugar, caster sugar and eggs in a large bowl; mix until smooth.
Mix flour, bicarbonate of soda and oats together in another bowl. Add to the butter mixture slowly; mix using an electric mixer until combined.
Spread 3/4 of the oat mixture into an ungreased traybake tin using a spatula. Press lightly into the bottom of the tin to make a base.
To make the fudgy chocolate filling: Combine chocolate chips, sweetened condensed milk, 75g butter and vanilla extract in a saucepan over medium-low heat. Stir until melted and smooth, about 2 minutes. Pour chocolate fudge over the oat base and spread evenly.
Drop tablespoons of remaining oat mixture over the chocolate fudge layer; press down lightly to flatten and swirl into the chocolate fudge layer.
Bake in the preheated oven until the top is lightly browned, 25 to 30 minutes; the chocolate should still be moist. Cool completely on a wire rack, about 45 minutes. Slice into squares.