Preheat oven to 350 degrees. Grease an oven safe skillet with the butter, making sure to cover the bottom and all the way up the sides.
In a medium bowl, whisk eggs. Add in milk and vanilla and whisk together until well combined. Whisk in 1/4 C of the sugar. Continue to whisk mixture for 2-3 minutes. Add the flour last and whisk until just combined.
Place your pan on the stove over medium heat.
Pour half of the batter into the warmed pan. Spread blackberries over the batter and top with remaining 1/4 C sugar. Pour remaining batter on top of berries.
Remove from stovetop and place into preaheated oven. Cook for 35-45 minutes until golden brown.
It is wonderful served warm, sprinkled with powdered sugar. Refrigerate any leftovers.