Preheat the oven to 180C/350F. Line a large baking sheet with parchment paper and set aside.
In a large mixing bowl, mix together the sugars, butter, and vanilla extract, until combined and smooth. In a separate bowl, mix together the boiling water with the baking soda.
Add the boiling water, before slowly fold through the flour and salt until combined. Add in the oats, and mix until a thick batter remains.
Scoop out heaping tablespoons of cookie dough onto the lined sheet. Bake the cookies for 10-12 minutes, or until the edges begin to go golden.
Remove from the oven and let cool on the sheets for 10 minutes, before carefully transferring to a wire rack to cool completely.
TO STORE: Leftover cookies can be stored at room temperature, in a sealed container. They will keep well for at least one week. Store them in the refrigerator if you'd like them to keep longer.TO FREEZE: Place cookies in a ziplock bag and store them in the freezer for up to 6 months.