Preheat oven to 350 degrees F. Grease a 9x13-inch baking pan.

Whisk together the flour, baking soda, and salt in a bowl and set aside.

With a stand mixer fitted with the paddle attachment or hand-held mixer, beat the butter and sugar on high speed until light and creamy, 3 minutes. Scraped the bowl down as needed. Add the eggs one at a time, followed by vanilla.

Add the flour mixture in 3 additions, alternating with the buttermilk. Mix on low just until combined.

Pour cake batter into the prepared pan and bake until golden and center bounces back when touched gently, 30-35 minutes.

While the cake is baking, make the fruit filling. Add the strawberries, raspberries, blueberries, sugar, lemon zest, water, cornstarch and salt in a small saucepan. Cook over medium heat until sugar is dissolved, berries have softened, and juices have just started to thicken, about 5 minutes. Remove from heat.

Use a wooden dowel or the end of a wooden spoon to poke holes about 1 inch apart all over the warm cake.

Spoon berry filling over the cake, encouraging it down the holes.

Cover with plastic wrap and chill completely in the fridge, 1-2 hours.

With a stand mixer fitted with the whisk attachment or hand-held mixer, beat the whipping cream, sugar, and vanilla to medium peaks. Spread over the top of the cake.

Serve immediately or store in the fridge until ready to serve. Can be made up to 1 day ahead.