Preheat oven to 180 C / Gas 4. Grease a baking tray or line with baking parchment.
In a bowl mix together 350g flour, baking powder, bicarbonate of soda and salt.
In a second bowl combine eggs and caster sugar and beat until light and creamy. Add honey, vanilla sugar and lavender and quickly stir together. Add to the flour mixture and knead until smooth and elastic, adding the 50g remainder of the flour if necessary to form a dough.
Divide the dough into two equal portions and form into two 30cm long loaves. Place onto the prepared baking tray, leaving a good amount of space between the two.
Bake for 30 minutes in the preheated oven, until golden brown. Set onto a cooling rack and let cool for 10 to 15 minutes. Turn oven down to 160 C / Gas 2/3.
When completely cool, slice each loaf into 2cm thick slices with a serrated knife. Place the biscotti slices back onto the baking tray.
Bake biscotti for another 7 minutes on each side.
Allow biscotti to cool at room temperature for a few hours before serving.