Get your oven ready by preheating it to 350 degrees F (177 degrees C). Put paper liners in 12 cups of a muffin pan. Line three cups in a second muffin pan with liners too. This recipe makes 15 muffins total. Put the pans to the side for now.

For the Crumb Topping

1. Mix the brown sugar, cinnamon, and melted butter together in a small bowl until they are combined.
2. Add the flour to the bowl. Use a fork to gently mix until crumbly pieces form. Do not mix too much or it will turn into a paste. Just mix until it looks like crumbs. It might seem like a lot of crumb topping, and it is! These muffins get a generous topping.
3. Put the crumb topping mixture in the refrigerator while you prepare the muffin batter. Keeping the crumbs cold helps them hold their shape better when baking.

For the Muffins

1. Whisk the flour, baking powder, baking soda, cinnamon, and salt together in a medium bowl. Set this bowl aside.
2. In a large bowl, use a handheld mixer or a stand mixer with a paddle attachment. Beat the softened butter and granulated sugar together on high speed until the mixture is smooth and creamy, about two minutes. Remember to scrape down the sides of the bowl as needed.
3. Add the eggs one at a time and beat on medium speed until they are mixed in.
4. Add the sour cream and vanilla extract. Beat until the mixture is combined and mostly smooth and creamy. It is okay if it looks a little bit curdled at this point. Scrape down the sides of the bowl as needed.
5. With the mixer running on low speed, add the dry ingredients mixture. Then add the milk. Try not to overmix the batter. You might need to switch to stirring with a spatula or whisking by hand to be sure there are no dry flour spots hiding at the bottom of the bowl. The batter will be thick.
6. Spoon about three Tablespoons of batter into each of the 15 prepared muffin cups. Fill them about two thirds of the way full.
7. Spoon the chilled crumb topping onto each muffin. Gently press the topping down so it sticks to the batter.

Bake the muffins for 22 to 24 minutes. You will know they are done when a toothpick inserted into the center comes out clean. It is normal for some crumb topping to fall off the sides as the muffins bake and expand, but most of it should stay securely on top.

Let the muffins cool in the muffin pan for five minutes. Then carefully move them to a wire cooling rack to cool completely. Let them cool for at least 15 minutes before adding any icing.

Make the Icing (if using)

1. Whisk the sifted confectioners’ sugar, two Tablespoons of milk or heavy cream, and vanilla extract together in a small bowl until smooth.
2. If you want thinner icing, add the third Tablespoon of milk or heavy cream. For thicker icing, add a little more confectioners’ sugar.
3. Drizzle the finished icing over the cooled muffins.

You can keep iced or plain muffins covered at room temperature for a few days. They also stay fresh in the refrigerator for up to one week.