Make the pastry. Using a food processor or your fingertips, rub the butter into the flour until it resembles fine breadcrumbs. Stir through until the mixture begins to clump together then work it into a ball of dough. Cover in clingfilm and chill in the fridge for at least 30 minutes.
Roll out the pastry on a lightly floured surface. Gently rest your tart tin over the pastry and trim the pastry to about 2.5cm wider than the tin. Drape your pastry over the tart tin, working it into the edges, and prick the base with a fork. Chill in the fridge for at least 15 minutes.
Preheat the oven to 200ºC/Fan 180ºC/ Gas 6. Line the pastry with scrunched baking paper and fill with baking beans. Bake for 15 minutes. Remove from the oven and use a sharp knife to trim the excess pastry away from the tin. Lightly brush the inside of the pastry with the reserved egg white and bake into the oven for a further 5-10 minutes .Remove from the oven and set aside.
Make the bramble jam. Stir together the blackberries, jam sugar, cordial and cornflour over a high heat. Bring to a boil then reduce the heat and simmer for about 5 minutes or until it registers 105ºC on a sugar thermometer. Remove from the pan and leave to cool to room temperature.
Make the frangipane. Cream together the butter and sugar until light and fluffy. Add in the flour, eggs, ground almonds and cordial and mix until combined.
Preheat the oven to 180ºC/Fan 160ºC/ Gas 5. Spread the cooled jam over the base of the pastry case. Top this with the frangipane and level off. Press the blackberries into the frangipane and scatter over the flaked almonds. Bake in the oven for 35 minutes. Remove from the tin to cool on a wire rack.
Make the icing by mixing the icing sugar with the cordial until a thick fluid consistency. Drizzle the icing over the top of the tart with bold strokes.