Grease an 8x8 inch baking dish with softened butter.

Arrange the cubed bread in the prepared dish.

In a large bowl, whisk together heavy cream, milk, sugar, egg yolks, vanilla extract, and salt until well combined.

Pour the custard mixture evenly over the bread cubes, ensuring all pieces are moistened. Gently press down on the bread to help it absorb the liquid. Let it soak for 15-20 minutes.

Preheat oven to 350°F (175°C). Gently fold in the mixed berries.

Bake for 40-45 minutes, or until the custard is set and the top is lightly golden. A knife inserted into the center should come out clean.

Remove from oven and let cool completely at room temperature.

Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, until thoroughly chilled and firm. This is crucial for clean slicing.

Before serving, use a very sharp knife to cut into portions. For the cleanest slices, wipe the blade with a damp cloth between each cut to prevent sticking and smearing. Serve chilled.