Preheat the oven to 425°F / 220°C. Butter a muffin tin. Before you pour in the batter, you'll need to place the prepared muffin tin in the oven to heat it until it is hot, about 5 minutes. I put the muffin tin in after I whisk the dry ingredients together. Try to time it so the empty tin is not left in the oven longer than necessary, and the finished batter is not resting longer than necessary.
In a small bowl, whisk together the flour, brown sugar, cinnamon and salt.
In a medium bowl, preferably one with a spout that you can pour out of, beat the eggs. Whisk in the milk and butter. The eggs and milk need to be at room temperature so the butter does not clump together.
Gradually, whisk the flour mixture into the egg mixture until combined. The batter will be liquid in texture.
Remove the muffin tin from the oven and quickly pour the batter into the cups, filling the cups to the rim. Bake for 20 minutes at 425°F / 220°C, then turn the oven down to 325°F / 165°C and continue to bake for another 40 minutes, until the popovers are completely golden brown and puffed up. Do not open the oven door until the popovers are just about ready, otherwise heat will escape and the popovers may deflate once they are out of the oven.
While the popovers are in the oven, prepare the cinnamon brown sugar topping by mixing the brown sugar and cinnamon together in a small bowl. Get the melted butter ready with a brush.
As soon as the popovers come out of the oven, brush the melted butter on top. Remove the popovers from the pan and quickly toss in the cinnamon brown sugar mix. Serve immediately.