Preheat oven to 400 degrees Fahrenheit. Whisk heavy whipping cream and egg in a bowl and set aside. In a large bowl, add the first 5 ingredients and whisk to combine. Add butter, and with both hands, rub the butter into the flour mixture being mindful not to squeeze or mash. Once the flour turns into a loose, coarse meal, start adding ¾ cup of the cream mixture, a small amount at a time, until a ball starts to form. Set aside remaining cream and the dough.
Coat the inside of a 10" cast iron skillet with butter. Set aside. Fill a 4 quart pot ¾ full with water. Place on stove and bring water to a simmer. Set a bowl with a colander to the side. Score an "X" at the bottom of each peach with a knife. Once the water is at a full simmer, place 3 to 4 peaches at a time in the water. Allow peaches to cook for 3 to 5 minutes. Carefully take peaches out and allow them to cool in the bowl set aside with colander. Repeat with remaining peaches. Once peaches are cool enough to handle, peel the skins off from the scoring site. Cut into quarters and place into bowl with the reserved liquids from cooling. Add the sugar, cornstarch, pinch of salt, and the scraped vanilla beans from the vanilla bean pod. Toss with fresh blackberries and fill prepared skillet. Place skillet on top of baking sheet lined with parchment paper and place in preheated oven. Allow the filling to heat and begin to bubble, about 10 to 20 minutes. Carefully remove from oven (mixture may spill over during process), and place the drop biscuits over the top of the hot cobbler. Coat with the remaining cream mixture and sprinkle with remaining 2 tablespoons of sugar. Place back in oven for 25 to 35 minutes, or until biscuits are golden brown.
Let stand for 30 minutes or up to one hour before serving. Best served with a scoop of vanilla ice cream.