Preheat oven to 190 degrees C. Line muffin trays with butter and flour, making sure to discard any excess flour from the tins after coating.
Sift together flour, baking powder, salt and cinnamon in a bowl. Set aside.
Toss together diced apples and 2 teaspoons of flour to coat apples in a separate bowl. Set aside.
Cream together butter and sugar until lightened in colour; about 3 minutes. Add the eggs, one at a time, taking care to fully incorporate before adding the other. Mix in the vanilla.
Gently fold in flour mixture, alternating with milk. Stir until just combined. Fold in the diced apples then scoop mixture into prepared muffin tins, filling about 3/4 full.
Bake until a toothpick inserted in the centre comes out clean; about 30 minutes.
Topping: While the muffins are baking melt the butter then allowing to cool slightly. Pour butter into a separate bowl sized easy for dipping tops of muffins. Mix together granulated sugar and cinnamon in a separate bowl then set aside.
Once muffins have baked, remove from the oven and allow to cool slightly in the muffin tray. Remove each muffin and dip first into the melted butter and then into the cinnamon sugar mixture. Place onto a plate to finish cooling.