Preheat oven to 350 degrees. Spray or grease the donut pan, really well.
Donuts:
Mix together the flour, salt, baking powder and baking soda. Melt the butter and add it to the sour cream, brown sugar, white sugar, milk, egg and vanilla and mix together.
Add the dry ingredients to the wet ingredients and with your mixer, mix these together only until they are incorporated. Do not over mix them or they will be tough.
Scrape the batter into a large plastic bag, squeeze the dough towards a corner and snip off the corner, so you can pipe the dough into the donut wells. Only fill each well about 3/4 full.
Streusel Topping:
Melt butter and add to a bowl containing all dry ingredients. Mix together with a fork until all topping has been coated with butter. It will be very crumbly.
Liberally top all 8 donuts with the streusel topping. You will want to pile the topping up on the donut batter, then press down slightly so it will stick onto the donut dough.
Place donuts into the preheated oven for 15 minutes or donut batter becomes golden brown.
Once you remove the donuts from the oven, allow them to cool for about 10 minute and then use a thin butter knife to go around the perimeter of each donut to loosen any stuck spots. Enjoy!!!
Notes
I store these in an airtight container and they stay fresh for approximately a week..