Preheat oven to 350 degrees °F.

In a large bowl or stand mixer with the whisk attachment, beat room temperature butter and sugar together until fluffy. Add eggs, one at a time, beating each egg into the mixture before adding another. Add vanilla extract and milk and continue to mix until fully incorporated.

In a separate bowl, mix together flour, baking powder, and kosher salt. Add to wet ingredients and fold together with a mixing spoon or silicone spatula until flour is fully incorporated. Spray a 9x5 loaf pan with oil and coat with flour. Add cake batter and bake for 1 hour to 1 hour and 10 minutes, until a toothpick comes out clean.

While cake is baking, combine blackberries, blueberries, raspberries, and strawberries in a large bowl. Add juice from one squeezed lemon and sugar. Mix together and chill in the refrigerator while cake is baking.

Allow cake to cool completely. Slice cake and cut into cubes.

To make the whipped cream, add cold heavy whipping cream, sugar, and vanilla extract to a stand mixer with the whisk attachment or a large bowl using a hand mixer. Beat on medium high until stiff peaks form, about 2-2 1/2 minutes. Be sure not to over mix.

Assemble the trifle by adding a layer of berries, then a layer of whipped cream, and then cake. Add more berries and whipped cream on top. Garnish with mint.