If using porridge oats and not oatmeal, blitz the oats in a food processor or Nutribullet until slightly grainy but not powdery.
Add your wholemeal flour, plain flour, baking powder and salt to your food processor and whizz briefly to mix together. Or mix together in a mixing bowl.
Cut your cold butter up into small cubes and dot over your flour mixture.
Pulse your food processor in blasts until the butter has all been evenly mixed in. Give the mixture a shake or a mix in-between to be sure the butter mixes in evenly.
Add your now flour-like oats (or oatmeal) and caster sugar and pulse again briefly until evenly mixed in.
Next, add your cold milk in one go and again pulse your mixer until it just begins to clump together.
Tip the mixture out onto your work surface and bring together into a ball.
Lightly flour your work surface and then knead your digestive biscuit dough for a minute, until it feels smoother.
Sprinkle a little extra flour over your rolling pin and then roll the dough out to a depth of about 4 millimetres and then cut out 24 biscuits. You will need to flour your work surface and re-roll your dough in between.
Cover your heavy-based baking sheet with a piece of greaseproof paper and then place your cut-out digestive biscuits onto the tray with a space of about 2 centimetres between each one.
Place the baking tray into your fridge for at least 10 minutes to set the dough and thus ensure the digestives don't spread when baking. Whilst they are in the fridge heat your oven to 170°C Fan/190°C/Gas mark 5/375°F.
Bake your digestive biscuits for about 12 minutes, until they are lightly golden all over.
Using a spatula or similar implement remove the digestive biscuits to a cooling rack and allow to completely cool.