In a medium bowl, combine the flour, salt, butter, sour cream, and egg. Mix until combined. Knead the dough several times, until smooth. Cover, and let sit for 30 minutes.
Next, in a large pot, add cold water and potatoes. Bring to a boil. Cook until the potatoes are tender, about 15 to 20 minutes until easily pierced with a fork.
Drain the potatoes, and return to the pot. While warm, mash with butter, cheese, salt, and pepper. Set aside to cool.
On a floured surface, divide dough in half. Working with one half, roll to 1/8 inch thick. Using a glass, about 3 inches wide, cut out circles, and re-roll, cutting as many circles as possible.
In the center of each circle, spoon 1 teaspoon of filling. Fold over and press down the edges to seal. Using a fork, press the edges to seal.
On a sheet tray, place the pierogies and cover with a towel. Prepared pierogies can be frozen for 3 months if desired.
In a large stockpot filled with water, bring to a boil. Once boiling, cook pierogies in batches for 1 to 2 minutes, or until they float.
Meanwhile, in a large skillet, brown the butter. Once pierogies are cooked, drain from pot into skillet, and cook until lightly golden brown. Remove to plate.
In the same large skillet, add spinach and sauté in the butter until lightly wilted. Season with salt, and plate with pierogies. Serve immediately.
