• Set the oven to 350º. Generously butter an 8 inch cast iron skillet, and set it in the oven to get hot. Depending on your oven, it will probably take about 10 minutes, which is about how long it takes to get the cake mixed up.

Mix up the topping

• Using a pastry cutter, or your fingers, blend the butter, brown sugar, flour and cinnamon together in a small bowl. Set aside.

Make the cake

• In the bowl of a stand mixer [or large mixing bowl if using an electric hand mixer], cream together the butter and sugar.
• Beat in the egg, vanilla and lemon zest.
• In a smaller bowl, whisk together the flour, baking powder and salt.
• Add the dry ingredients and the buttermilk alternately - half the flour, all the buttermilk, then the other half of the flour, mixing well after each addition.
• Mix the 1 tablespoon flour and the blueberries together, and fold into the cake batter.
• Carefully remove the hot skillet from the oven, and spread the batter evenly in it.
• Scatter the topping mixture evenly over the batter.
• Bake for around 25 minutes, until a toothpick or skewer comes out clean, when inserted near the center.
• Let cool for 10 minutes, and serve warm, plain or with a sprinkle of powdered sugar.
• Remove leftovers from pan, and store in the fridge for up to several days.