Preheat oven to 350 degrees (F). Spray your donut mold with non-stick spray and set aside.
In a medium-sized bowl whisk together the flour, light brown sugar, salt, baking powder, and cinnamon.
In a separate bowl mix the wet ingredients, make sure the melted butter has cooled or it will curdle the sour cream and cook the egg.
Gently fold the wet ingredients into the dry ingredients until they are well combined. The dough will be much thicker than you think it will be, so don’t be alarmed. Also, be sure not to overmix your dough or your baked donuts will be tough.
Spoon or pipe the batter into your molds and fill them up to the top. If you’re using a silicone mold, place it on a cookie sheet before you put them in the oven, this will make it easier to get them in and out of the oven.
Sprinkle the streusel over the top of each donut.
Place your donuts in the pre-heated oven. They usually take about 15-17 minutes to bake, but I started checking them around 10 minutes to see if the pan needs to be turned. You’ll know they are finished when you lightly press on a donut and it springs back.
Remove from oven and allow to cool in the pan for 10 minutes and then remove from pan and allow to cool completely before icing.