Preheat the oven to 190C / Fan 170 / Gas Mark 5. Place the porridge oats into a blender and blitz for about 20 seconds until a fine powdery flour is formed. 

In a large bowl, mix together the blitzed oats, bicarbonate of soda, cream of tartar, light golden brown sugar and 50g of the milk chocolate chips. 

Stir in the mashed banana, melted butter and vanilla extract. 

Divide the cookie dough into 12 equal portions, then place onto a baking tray lined with baking parchment and flatten slightly. 

Bake for 15-20 minutes until the cookies are firm to the touch. Allow to cool on the tray for five minutes, then transfer to a wire rack. 

Melt the remaining chocolate chips following the packet instructions and drizzle it over the cookies to finish.