Line a loaf pan with parchment paper and preheat the oven to 350°F (180°C)
In a large bowl cream the butter, sugar and icing sugar. Beat for about 1 minute until the mixture looks creamy and smooth.
Add the eggs, yoghurt and flax seed to the butter and sugar, beat them just until foamy and well combined.
To a medium bowl measure out the buckwheat flour, sorghum flour, tapioca starch, coconut flour, shredded coconut baking powder and salt.
Add half of the flour blend to the egg mixture, and beat it until just combined. add the other half of the flour and fold it in. Stir in the lemon juice and zest.
Pour the mixture into a loaf pan and smooth the top with a spatula.
Bake at 350°F (180°F) for 35 to 40 minutes or until an inserted toothpick comes out clean.
Cool in the pan for 10 minutes, remove the loaf from the pan and cool it on a cake rack while preparing the icing.