Preheat your oven to 180°C (350°F) for conventional or 160°C (320°F) for fan-forced. Grease and line a quarter sheet pan (9 by 13 inches) with baking or parchment paper.

In a large mixing bowl, combine the flour, sugar, baking powder, and salt. Whisk gently to combine.

Add the chopped unsalted butter to the bowl and beat with an electric mixer on medium speed until the butter resembles small pea-sized pieces.

Add the lemon zest, eggs, milk, and vanilla extract. Start on low speed and gradually increase to medium. Add the freshly squeezed lemon juice and combine until smooth.

Gently fold through the chopped blackberries by hand, ensuring even distribution without breaking them apart.

Pour the cake batter into the prepared pan and bake for approximately 30-35 minutes, or until a skewer inserted into the middle comes out clean. Allow the cake to cool in the pan completely.

Preheat the oven again to 180°C (350°F) and line a medium-sized cookie sheet with baking paper. In a mixing bowl, combine flour, sugar, cornflour, salt, and lemon zest. Whisk thoroughly.

Pour the melted butter over the dry ingredients and add vanilla. Stir until the mixture resembles wet sand. Spread the crumbs onto the prepared tray and bake for 10 minutes. Let them cool completely.

In a large bowl, beat unsalted butter with an electric mixer on medium speed until pale and creamy.

Melt the white chocolate in the microwave in 20-second bursts, stirring in between, and set aside.

Add vanilla, half of the icing sugar, and one tablespoon of milk to the butter, mixing until just combined. Gradually add the remaining icing sugar and beat until smooth. Mix in the melted chocolate, ensuring it’s fully incorporated.

If the frosting is too thick, add an extra tablespoon of milk.

Carefully remove the cooled cake from the pan and peel off the paper. Use a six-inch cake ring to cut out three layers, yielding two full rounds and two semicircles.

Place the cake ring on a cake board or plate lined with acetate. Press the two semicircles onto the bottom firmly. Pour 2-3 tablespoons of the milk soak over the first layer.

Spread one-third of the buttercream over the layer evenly. Add 2-3 tablespoons of blackberry jam and swirl it into the buttercream. Scatter a handful of lemon cookie crumbs.

Repeat the process with another cake layer, pressing down, and soak, buttercream, jam, and crumbs again. Finish with the final cake layer.

Top the final layer with the remaining buttercream, spreading it evenly. Add more jam and sprinkle over the rest of the cookie crumbs.

Place the completed cake in the fridge for at least two hours to set.

When ready to serve, carefully remove the acetate and decorate the top with fresh blackberries.