Preheat oven to 300 degrees F. Grease each cavity in your mini loaf pan with cooking spray. If you're using a 9x5-inch loaf, spray that with the cooking spray. Set aside.
In a medium bowl, mix together the all-purpose flour, salt and baking powder. Set aside. In the bowl of a stand-up mixer, mix together the vanilla bean caviar and sugar until the little speckles of vanilla bean are mixed throughout.
Add the softened butter to the bowl of a stand-up mixer. With the paddle attachment on the mixer, beat the sugar and butter together until light and fluffy, about 3 minutes. Mix in one egg at a time, being sure each egg is incorporated before adding another.
Add the flour mixture to the butter mixture, alternating with the milk, beginning and ending with the flour. Pour the pound cake batter into the prepared the mini loaf pan cavities or the 9x5-inch loaf pan and transfer to the oven to bake. The mini loafs will take about 25 to 30 minutes. They should appear domed and golden brown at the edges. Test their doneness by inserting a skewer into the center; it should come out clean.
If you're using a 9 x 5-inch loaf pan, the pound cake will take 50 to 55 minutes, until a skewer inserted into the center comes out clean. Allow the cakes or cake to cool for 5 minutes in the pan and then invert onto a cooling rack.
While the cakes are cooling, turn your oven up to 400 degrees F. Line a baking sheet with parchment. Lay the grapes onto the baking sheet and sprinkle with salt and pepper. Transfer to the oven to roast for 10 minutes. I like the grapes to be slightly softened and juicy but not so cooked they fall apart. Set aside to cool. Using a pair of scissors, clip of baby vines of two to three grapes for mini loaf pans, larger vines of six to seven grapes for a larger loaf.
To make the frosting, add the sifted powdered sugar into the bowl of a stand-up mixer. Add the lebneh and beat until very smooth.
To assemble the cakes, frost them as you like. You have options! You can transfer the frosting to a piping bag and pipe it on how you like. I sort of did a triangle situation in the corner. If you do this, serve the cakes with a side of frosting; you'll have extra and it's delicious. However you frost it, garnish it with a mini vine of grapes.