Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a bowl, whisk together flour, baking powder, baking soda, and salt.
In another bowl, cream butter and sugar until light and fluffy. Beat in eggs one at a time, then mix in lemon zest and juice.
Alternately add flour mixture and buttermilk to the butter mixture, starting and ending with flour, until just combined.
Gently fold in mixed berries. Divide batter evenly between pans and bake for 25-30 minutes, or until a toothpick inserted comes out clean. Cool completely.
For frosting, beat cream cheese, powdered sugar, and vanilla until smooth. Spread between layers and over the cooled cake. Garnish with extra berries and lemon slices.