Preheat oven to 180 C. Grease a 20cm x 30cm slice pan; line base with baking paper, extending paper 5cm over long sides.
Place syrup, dates, salt and the water in a small saucepan over low heat; simmer for 5 mins or until dates are soft.
Stir soda into date mixture; transfer to a food processor, process until smooth. Return date mixture to pan; add butter, stir over medium heat until butter melts. Transfer mixture to a large bowl; cool 5 mins.
Whisk in eggs, one at a time. Stir in sifted cocoa, ground hazelnuts, flour, sour cream and hazelnuts. Spread mixture into pan; level top.
Bake for 30 mins or until a skewer inserted into the centre comes out with moist crumbs attached. Cool in pan before dusting with extra cocoa and cutting into squares.
Despite its name, buckwheat is unrelated to wheat and is, in fact, a seed or pseudo-cereal, making it safe for those on coeliac diets. It is also high in fibre and protein.