Preheat the oven to 180°C/356°F.
Peel the sweet potatoes, cut in cubes, place in a pan with enough water to cover them, bring to boil and simmer for 15 minutes over low heat. Drain well and set aside to cool.
Add the sweet potatoes, soaked dates and coconut oil in the food processor, and mix to obtain a puree.
Add almond meal, buckwheat flour, cacao powder, vegan milk, salt and baking powder, and process to create an smooth batter.
Melt the vegan chocolate and pour over the mixture, then pulse two times, to incorporate.
Pour the mixture into a lined baking pan and cook for 25 minutes.
Remove the baking pan from the oven, and set aside to cool for 10 minutes.
Remove the brownie from the pan, let cool completely before cutting in squares. Store in the refrigerator for a maximum of four days.