Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish with butter.
Spread the softened butter on one side of each slice of bread. Cut the bread into triangles or quarters.
Arrange half of the bread slices, buttered side up, in the greased baking dish. Sprinkle half of the raisins or currants over the bread.
Layer the remaining bread slices on top, again buttered side up, and add the rest of the raisins or currants.
In a mixing bowl, whisk together the milk, heavy cream, eggs, sugar, vanilla extract, cinnamon, and salt until well combined.
Pour the custard mixture evenly over the layered bread in the baking dish. Press down gently to ensure the bread absorbs the liquid.
Let the mixture sit for about 10 minutes to soak in.
Bake in the preheated oven for 35-40 minutes, or until the top is golden brown and the custard is set.
Allow to cool slightly before serving. Dust with powdered sugar if desired.