In a medium bowl, whisk the eggs. Whisk in the sugar, salt, and vanilla. Gradually whisk in the half-and-half and the buttermilk. Pour the mixture into a shallow baking dish. Working in batches, if necessary, place the bread in the dish and let soak, then turn and soak on the other side.
Cook the soaked bread on a grill or buttered griddle until golden brown, then turn and repeat on the other side.
Meanwhile, prepare the grapefruit sections.
In a saute pan, add the maple syrup and bring to a boil. Place the grapefruit sections in the hot syrup and cook on 1 side for 30 seconds, then flip them over to cook and warm through on the other side. Pour the grapefruit and syrup over the French toast then top with dollops of mascarpone.