For the cake: Place a 10-inch cast-iron skillet in the oven. Preheat the oven to 375 degrees F.
Whisk together the flour, brown sugar, baking powder and salt in a large bowl. Add the blueberries and toss them gently in the flour mixture; this will help keep the blueberries from sinking to the bottom of the cake.
In a separate bowl, whisk together the buttermilk, vanilla and eggs. Remove the skillet from the oven and add the oil and butter to the hot skillet. Let the butter melt completely, then pour the oil and butter into the buttermilk mixture and whisk together, fully incorporating the hot fats. Add the liquid mixture into the dry mixture and stir gently until combined and most lumps are gone. Pour the batter into the hot skillet. Bake until the cake is fluffy and golden brown and a toothpick comes out clean when inserted in the middle, 30 to 35 minutes.
For the white pancake syrup: While the pancake cooks, add the corn syrup, vanilla and cinnamon to a small saucepan. Stir together and cook over low heat until the syrup is warm and pourable.
Slice the warm pancake in wedges and serve with the white pancake syrup.