Preheat the oven to 160c fan or 180c
Grease and line a 21cm spring form cake pan with baking paper
Add the sugar and dried lavender in a food processor and pulse until the lavender has broken up and blended with the sugar
Add the lavender sugar in the bowl of a stand mixer fitted with the balloon whisk attachment and add the butter and whip for 2 minutes, then scrape around the sides of the bowl with a spatula and whisk again for a further 3 minutes or until pale, light and fluffy. scrape around the sides of the bowl again ready for the next step. ( alternatively make in a bowl using a hand held electric whisk, if you don't have an electric whisk you can make it by hand with a hand held whisk but this will take more time)
Add one egg and turn up the speed and whisk for 2 to 4 minutes, turn off mixer and scrape the sides of the bowl and mix again, then add the next egg. Continue this process of whisking for 2 to 4 minutes and scraping the bowl before you add the next egg. once all of the eggs have been added, add the vanilla and mix for 1 minute.
Remove bowl from stand mixer and sift in the flour and salt and fold in using the spatula.
dust blackberries with flour and fold in 3/4 of them and pour into the lined baking pan, then press in the rest on the top.
Put the cake in the oven for 15 minutes, just long enough to slightly bake it making it safe to add the topping, while its in the oven make the frangipane top