Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add this dry mixture to the wet mixture, alternating with the milk, starting and ending with the flour mixture. Mix until just combined.

Gently fold in the mixed berries and lemon juice, being careful not to break the berries too much.

Divide the batter evenly between the prepared cake pans. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cakes to cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

Once cooled, layer the cakes with whipped cream or frosting of your choice, and dust the top with powdered sugar before serving.