Preheat the oven to 375°. In a large skillet, melt 1 tablespoon of the butter. Add the pound cake and toss to coat. Spread the cubes on a baking sheet and bake for 15 minutes, until toasted.
Meanwhile, wipe out the skillet. Add the remaining 3 tablespoons of butter and the nectarines and cook over high heat, stirring occasionally, until the nectarines are slightly softened, about 5 minutes. Add the brown sugar and stir to melt. Remove the skillet from the heat. Add the bourbon and lemon juice. Tilt the pan slightly and carefully ignite the alcohol. Cook over moderate heat until the nectarines are softened and the sauce is slightly thickened, about 10 minutes.
Scoop the ice cream into bowls and spoon the nectarines and sauce on top. Garnish with the croutons and serve right away.