1. Grease a 9x13", deep baking dish with 1 Tbs. softened butter. Arrange the bread slices in the baking dish in two rows, overlapping the slices.
2. In a large bowl, lightly beat the eggs. Whisk in the cream, milk, granulated sugar, and vanilla. Evenly pour the mixture over the bread. Cover and refrigerate overnight.
3. Position a rack in the upper third of the oven and preheat the oven to 375 degrees F. Remove the baking dish from the refrigerator and let stand at room temperature for 30 minutes.
4. Place the brown sugar, pecans, flour, and salt in a medium bowl. Grate the stick of cold butter on top and mix until blended and crumbly. Sprinkle the mixture over the bread.
5. Place the baking dish in the oven and bake, uncovered, until browned and the inside is set but soft, 35-40 minutes. Cool slightly.
6. Dust with powdered sugar, if you choose. Serve warm, with maple syrup if desired.
7. The casserole is best served the day it is made. Store leftovers in a covered container in the refrigerator for up to 2 days.