In a small saucepan over medium-low heat, add the raisins and pour over the bourbon. Stir to coat and when it’s about to simmer, remove from the heat; let the fruit macerate for 30 minutes.
Scoop up the soaked raisins into a bowl using a slotted spoon and reserve the liquid for later.
Preheat oven to 350º and grease a 3-quart baking dish with butter; set aside.
In a medium saucepan over medium-heat, add butter, half-and-half, milk, sugar and ground sea salt.
Stir until the sugar dissolves and heat the mixture until the bubbles begin to form around the edges and the milk starts to steam – make sure it doesn’t simmer.
Remove from the heat and temper the beaten eggs by slowly adding a little amount of the hot mixture at a time, whisking constantly.
When the egg temperature is brought up, add the mixture to the saucepan, whisking continuously.
To this, add pure vanilla extract, ground cinnamon, freshly ground nutmeg and the reserved bourbon. Whisk the ingredients until nicely blended and set aside.
Add half of the bread cubes to the prepared baking dish and half of the soaked raisins; repeat the layers.
Pour over half of the custard. Press down gently with clean hands to compact the bread mixture before pouring the rest of the custard over.
Transfer the casserole to the preheated oven and bake for 40 minutes.
About 25 to 30 minutes through cooking, cover with foil to prevent the top from getting too dark; continue baking for the remaining time.