Grease the inside of a 6-quart slow cooker crock with butter or nonstick spray.
Place the cubed bread in the prepared slow cooker. If using raisins, sprinkle them evenly over the bread.
In a large bowl, whisk together the eggs and granulated sugar until combined. Whisk in the milk, heavy cream, melted butter, vanilla extract, cinnamon, and salt until the custard is smooth.
Pour the custard mixture evenly over the bread in the slow cooker. Gently press the bread down with a spatula to ensure all the pieces are saturated. Let it sit for 15 minutes to allow the bread to absorb the liquid.
Cover the slow cooker and cook on LOW for 4 to 4.5 hours, or on HIGH for 2 to 2.5 hours.
The bread pudding is done when the center is set and a thermometer inserted into the middle reads at least 160°F (71°C). The edges will be golden brown and pulling away from the sides.
Turn off the slow cooker and let the pudding rest for 20-30 minutes before serving. It will firm up as it cools.
Serve warm, scooped into bowls. Dust with powdered sugar and top with fresh berries if desired.