Preheat the oven to 155°C / 135°C Fan / 310°F (155°C) / Gas Mark 3. Butter a 23cm / 9-inch springform cake tin and line the base with baking parchment.
Spread the ground and flaked almonds on separate baking trays. Toast for 15–20 minutes, turning every 5 minutes, until golden brown.
Set aside to cool completely.
To make the cake, combine the eggs, oil, and 250g (8.8 oz) of sugar in a large mixing bowl.
Using an electric or stand mixer, whisk for several minutes until the mixture is very thick, pale, and voluminous.
Gently fold in the flour, toasted ground almonds, baking powder, almond extract, and salt until just combined.
Take 75g (2.65 oz) of the blackberries, slice them in half, and gently fold them into the finished batter.
Spoon the batter into the prepared tin and bake for 55–60 minutes, or until a skewer inserted into the centre comes out clean.
Let the cake rest in the tin for 10 minutes before turning it out onto a wire rack to cool completely.
Meanwhile, make the crumble.
In a bowl, rub the cold butter into the flour, sugar, cinnamon, and salt with your fingertips until it resembles breadcrumbs. You can also pulse it in a food processor.
Spread evenly on a baking tray and bake for 30–35 minutes, turning every 10 minutes, until golden and crisp. Allow to cool.
For the purée, place 200g (7.1 oz) of the blackberries in a small pan with the remaining 25g (0.88 oz) of sugar and 4 tbsp of water.
Cover and cook over a low heat for a few minutes until the berries have softened.
Blend with a hand blender, then pass through a fine sieve to remove the seeds.
Set aside to cool.
To assemble, spoon the cooled blackberry purée over the top of the sponge. Scatter generously with the crumble topping and the toasted flaked almonds.
Arrange the remaining fresh blackberries on top and finish with a dusting of icing sugar, if desired.
Serve with a dollop of mascarpone.
Don't rush the whisking!
Beating the eggs, oil, and sugar until very thick and pale is the secret to a stable, non-greasy batter and a light, fluffy sponge.
