Preheat the oven to 350°F (175°C). In a medium bowl, whisk together the flour, sugar, baking powder and salt.
Melt the butter in a 10–12-inch cast iron skillet. Transfer to a bowl and whisk in the milk and vanilla. (If the milk is too cold, the butter will solidify — warm the mixture if needed to re-melt the butter.)
Add the milk mixture to the dry ingredients and stir gently until just combined. The batter will be lumpy — don't over-stir, or the cake won't be tender.
Scrape the batter into the cast iron skillet. Toss the blackberries with the 2 tbsp granulated sugar, then scatter over the batter. Bake about 40 minutes, until deeply golden and springy when pressed. Dust with confectioners' sugar, if desired.
Notes
Lumpy batter is good: Stir just until combined for a tender crumb; over-mixing toughens it.Watch the butter: If the milk is cold the melted butter can seize — warm the mixture to re-melt it before combining.Berries on top: They sink and "buckle" the cake as it bakes — that's the signature look.Cast iron bonus: The skillet gives deeply golden, slightly crisp edges.