Preheat the oven to 350ºF.
Grease and line 3 8-inch cake pans with parchment, ensuring the sides are well coated to prevent sticking.
In a medium bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until evenly combined.
In the bowl of a stand mixer, beat the softened butter until smooth and creamy. Add the dry ingredients and mix until the mixture resembles fine crumbs. This reverse-creaming method coats the flour in fat, resulting in a softer, more even crumb.
In a separate jug, whisk together the milk, oil, eggs, and vanilla.
With the mixer on low speed, gradually pour in the wet ingredients. Once incorporated, increase to medium speed and mix for about 1 minute until smooth. The batter should be fluid and cohesive. Avoid over mixing at this stage.
Divide the batter evenly between the pans.
Bake for 28–32 minutes, or until: the tops are lightly golden, the centers spring back when pressed, a toothpick inserted into the center comes out clean.
Remove from the oven then cool in the pans for 10 minutes, before turning out onto wire racks to cool completely.
Make the berry filling: In a bowl, stir together the jam and water until loosened and smooth.
Add the strawberries, raspberries, blueberries, and lemon zest and juice. Gently fold to combine, keeping the fruit pieces intact.
Let the mixture sit for 10–15 minutes until lightly macerated. The filling should be glossy and slightly syrupy, but not overly wet.
Make the Chantilly cream: In a large bowl, beat the mascarpone and cream cheese together until completely smooth and lump-free. This step is essential for a silky frosting.
Add the powdered sugar, vanilla, lemon zest, and salt. Mix until fully combined.
In a separate bowl, whip the heavy cream to stiff peaks.
Fold one-third of the whipped cream into the mascarpone mixture to loosen it, then gently fold in the remaining cream in two additions until light and airy. Do not over mix, as this can cause the frosting to lose structure.
Place the first cake layer onto a serving board. Spread a layer of Chantilly cream then pipe a border around the edge which will serve as a dam for the filling. Spoon over a thin, even layer of berry filling, leaving a small border around the edge. Lightly press the berries into the cream.
Repeat with the second layer.
Place the final layer on top apply a thin crumb coat over the top and down the sides of the cake then chill for 20–30 minutes until set.
Finish with a lightly textured final coat of frosting all over the entire cake.
Top with fresh berries and edible flowers.
Slice, serve and enjoy!