Prepare an 8 x 8-inch baking dish by lining it with parchment paper or greasing it with lard or cooking spray.
In a double-broiler, melt chocolate over low heat, stirring it often to prevent it from burning. When it is melted, remove it from the burner and stir in the avocado oil until combined. Let cool.
(Optional) While the chocolate is melting, chop and toast nuts in a dry frying pan stirring often to prevent them from burning. Set aside to cool.
Place the egg and egg whites in a mixing bowl and beat with an electric mixer until the eggs are frothy. Beat in the sugar until smooth.
Gradually beat in the melted chocolate and oil mixture until the batter is smooth and glossy.
In a separate bowl, combine the buckwheat, salt, and baking soda. Stir the dry ingredients into the chocolate batter to blend.
Gradually fold in the pure vanilla extract and the hazelnuts if using.
Pour into a prepared pan and smooth the top. Bake in the preheated oven for 20-30 minutes or until the batter appears set. Don't overbake.
Let cool before slicing.
Notes
For the best results, I recommend using high-quality baking chocolate. I use Bernard Callebaut No 811 Belgian Dark Chocolate.