Preheat the oven to 350ºF. Place a 9" oven safe skillet in the oven while it's heating up. Once heated, remove the skillet from the oven and melt 2 tbsp of butter in it. Swirl the butter and let cool just slightly.
In a bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
In a separate bowl, whisk together sugar, eggs, 4tbsp melted butter, vanilla, lemon zest, and buttermilk.
Add the dry ingredients into the wet and stir gently until just combined.
In a small bowl, mash the blackberries with 1tsp sugar. Leave the blackberries chunky rather than fully juiced.
Pour batter into the hot buttered skillet. Dollop the blackberries all over the top of the cake. Swirl them using a toothpick.
Bake 28-32 minutes until the center is set and the edges are golden.
Let cool before slicing. Serve with whipped cream and fresh blackberries. Enjoy!