Preheat an oven to 350F. Place unsalted butter in a 12-inch cast-iron skillet, then place it in the oven while you make the batter.
Place raspberries in a mixing bowl. Add granulated sugar, lemon zest, then toss well to thoroughly mix.
In a separate bowl, combine all-purpose flour, granulated sugar, baking powder, and Kosher salt, then whisk well.
Pour in milk and vanilla extract, then whisk until smooth.
Carefully remove the skillet from the oven, then pour the batter in all at once. Scatter the sugary raspberries all over the top. Sprinkle with chocolate chunks. Serve heaping scoops warm with powdered sugar, whipped cream, or other suggested toppings!