Steam the sweet potatoes until they are soft. I cooked mine in the microwave then removed the skin.
Pit the dates if they have stones in. Chop roughly then put in a food processor with the sweet potatoes. Blend to a puree.
Put the date and sweet potato puree in a bowl with all the other ingredients and stir well. The mixture will be quite stiff.
Transfer to a square tin lined with baking paper.
Bake in the oven at 180C for 40-50 minutes. A skewer should come out dry when they are ready.
Let the brownie cool in the tin for about 10 minutes before removing and transferring to a wire rack.
While the brownies are cooling, put all the ingredients for the icing in a small bowl. Heat in the microwave for a minute then stir together. Let it cool in the fridge. If it's still too runny when you are ready to use it, put it in the freezer for a short time to firm up.
Cut the brownies into squares and spread the icing on top.
They will keep well for about 4 days in an airtight tin.
Nutritional information is approximate and a guideline only.