Preheat oven to 400F/200C and line a large baking tray with a silicone mat (or baking paper if you don't own a silicone mat)
Whisk all-purpose flour, salt, sugar, baking powder, lemon zest and lavender together in a large bowl.
Grate butter with a cheese grater, and add to the flour mixture.
Use your hands to rub the butter and flour mixture together, until it resembles breadcrumbs or a course flour like almond flour.
Combine heavy cream and egg in a bowl, and whisk together.
Add to the flour and butter mixture, and use a knife to gently cut the batter to mix it together.
Gently use a spatula to fold in the blackberries until the dough is well combined, but be careful not to overmix the dough.
Turn the dough onto your silicone mat, and shape into a round, 8"/20cm disc. The dough should be around 1.5"/4cm thick.
Cut the dough into 8 equal sized triangles and gently pull them apart from each other so they don't touch.
Brush the triangles with a little bit of heavy cream or milk to help them crisp up.
Place in the oven for 20-25minutes. They should be starting to lightly brown on the top and cooked in the middle, but still soft. Set aside to cool.