Preheat oven to 350°F (175°C). Grease and flour a mini muffin tin.
In a bowl, cream together butter and sugar until light and fluffy.
Add eggs one at a time, beating well after each addition.
Mix in vanilla extract and lemon zest.
In another bowl, whisk together flour and a pinch of salt.
Gradually add flour mixture to the butter mixture, alternating with milk, beginning and ending with flour mixture.
Fold in mixed berries gently.
Spoon batter into prepared muffin tin, filling each cup about ⅔ full.
Bake for 15-20 minutes or until a toothpick inserted into the center comes out clean.
Let cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
In a small bowl, whisk together powdered sugar and lemon juice to make the glaze.
Drizzle glaze over cooled petit fours.