Place the oats in a small bowl. Cover with the boiling water and set aside to gel and cool until just slightly warm to the touch (or cooler).
In a medium bowl, cream together the margarine and sugars until smooth - you don't need to get 'light and fluffy' for this. Add the cooled oat goop, vanilla & salt and mix together until combined. It may look curdled at this point - this is fine.
Add both flours and your baking powder & bicarbonate of soda. Stir until you get a soft dough.
Roughly chop your dark chocolate (you want the chunks to be a variety of sizes but max. around 2 cm) - using a serrated/bread knife here helps prevent the chocolate flying all over the place. Scrape all the chocolate into the bowl and mix until combined.
Take heaped tablespoons of dough (around 45g/1.5 ounce) and roll into balls. Place into a container or on a tray and chill for at least 1 hour (preferably overnight) or up to 5 days.
When ready to bake, line a baking tray with baking paper and preheat your oven to 200°C fan (400°F) or 220°C non-fan (420°F).
Place balls of dough onto your lined baking tray, spacing them about 5cm (2 inches) apart. Sprinkle with a bit of flaky salt, if using, and bake for 6-10 minutes until the edges are set but the centre is still a bit squishy & gooey. Allow to cool on the tray for 1-2 minutes before transferring to a wire rack to cool completely.
Repeat with the remaining dough, as needed, until all the cookies are baked.
Note: if you want the freeze the dough, stop after you've completed step 5. Freeze the dough balls on the tray until solid then pop them into a resealable plastic bag. Place your bag of cookie dough balls into the freezer. You can bake them from frozen, you may just need to add 1-2 minutes to the baking time.