Carefully wash and pick through the blueberries (checking for twigs or rocks). Drain and pat them dry (I do this by pouring them over a towel lined baking sheet). Set aside.
Whisk together all the dry ingredients in a medium bowl. Set aside
Add the milk and vanilla and stir to form a thick batter. Set aside.
Divide the butter between 8 ramekins (or one big 9-inch ceramic baking dish). Place the pan(s) in the oven for 7 or 8 minutes, or until the butter is completely melted and the pan is hot.
Remove the pan(s) from the oven, and carefully pour the blueberries over the melted butter (½ cup per ramekin, or all in the bigger cake pan). Top with berry layer with the cake batter evenly.
Return the pan(s) to the oven and let them bake until the cake is golden brown and baked through (check with a cake tester). The baked blueberries will bubble and release some juice. The individual ramekins take about 35 minutes, while the larger cake pan takes about 45 minutes.
Remove the pan(s) from the oven, and let cool slightly before serving.