Preheat oven to 350°F.
Cube bread then place in a large bowl.
In another bowl, beat eggs, milk, half & half, sugar, salt, and vanilla. Pour over the bread and allow to sit 5 to 10 minutes
In another small bowl, combine with a fork, softened butter, brown sugar, and pecans. This mixture will have the consistency of wet sand.
Pour half of the bread mixture into a greased 8x8 inch pan. Top with half of the pecan mixture. Spoon remaining bread mixture over and top with remaining pecan mixture. Press down into pan slightly. Pan will be really full.
Top with remaining pecan mixture.
Place pan on a cookie sheet with a 1-inch edge to catch any mixture that may boil over. Bake at 350°F 45 to 55 minutes.
The center will be slightly jiggly but will set when cool. Allow to cool 20 to 30 minutes before serving so that the slices will hold together.