This one is no great secret. It's a slight modification of the recipe that used to be on the back of the oatmeal box. They're putting a different one on now, but I'm sticking with this one.
Cups are US cups, the volume of which is the same as 8 ounces of water.
Some ingredients have different names in the US and UK. US names first, UK names after the slash.
OATMEAL CHOCOLATE CHIP COOKIES
1-1/4 cups butter
3/4 cup firmly packed brown/muscavado sugar
1/2 cup granulated/caster sugar
1 egg
1 tsp vanilla
1-1/2 cups all purpose/pastry flour
1 tsp baking soda/bicarbonate of soda
1 tsp salt
2 cups chocolate chips
3 cups rolled oats
Optional: add 1 cup grated coconut or 1 cup chopped nuts (I like pecans).
Combine ingredients in the order listed, ensuring that dough is well mixed before adding each new ingredient. Scoop into cookie shapes on parchment-lined or buttered cookie sheet. They'll spread, so give them some space.
Bake at 350 F/180 C in preheated oven for 8-10 minutes. Cool, enjoy.
Note: My mother used to bake something like this as a solid block in a brownie pan, which was then served in squares. No idea how long that would take to bake, but I'm sure you can figure it out if you're interested.