Preheat the oven to 170°C and line 2 baking trays with baking paper and set aside.
In a mixing bowl, mix the oats, flours, 100 g of the sugar and salt.
Place the oil, honey and vanilla in a saucepan and gently heat through.
Place the bicarbonate of soda and boiling water in a small mixing bowl and stir until combined. Pour into the oil mixture, and gently stir, don’t be alarmed as it will froth up. Pour the mixture over the oat mixture and mix well to combine.
Roll small amounts of the dough into 30 g walnut-size balls and place spaced out on the prepared baking trays, use the palm of your hand to slightly flatten. Bake for about 8 – 10 minutes.
Carefully transfer the biscuits to a cooling rack and leave to cool completely, the biscuits might seem soft but as they cool down, they will harden. At this stage you can store the biscuits in an airtight container until you are ready to serve them. (May store for up to 2 weeks)
Once the biscuits are cooled and ready to serve, mix the mango purée, mascarpone, and remaining castor sugar well together and spread over the biscuits with a spoon. (May pre-prepare the mango mascarpone and store in the fridge in an airtight container for up to a week)