Preheat the oven to 200C/180C Fan/Gas Mark 6. Line a 9x13" pan with greaseproof paper (HACK: use paint-free bulldog clips to keep the baking paper attached to the tray).
In a large bowl, or the bowl of a stand mixer, cream the butter and brown sugar for about 3 minutes on a high speed (with an electric mixer). Add the eggs and vanilla and mix again.
Add in the oats, flour, baking powder and salt, and fold everything together into a sort of cookie dough/paste. Press 2/3rds of this into the lined tin (clean hands or a slightly damp wooden spoon work best - the mixture will be sticky).
Make the fudge by adding all ingredients (butter, chocolate and condensed milk) to a medium saucepan over a medium-low heat. Stir constantly until it has all melted together. Pour this over the oat layer in the tin.
Scatter over the remaining oat cookie mixture in random blobs. The fudge should be exposed in places! Place in the oven and bake for 25-30 minutes, until the top is golden brown. Let cool on a wire rack before slicing!
