Preheat the oven to 300.

In a large bowl, using an electric mixer, beat the butter with the sugar and salt until smooth and fluffy.

Mix the baking soda with the vanilla, then add to the butter mixture.

Add the flour, 1 cup at a time, mixing well between additions.

Gently stir in the Rice Krispies.

Lightly butter 2 baking sheets.

Spoon 2-tablespoon-size mounds of cookie dough onto the prepared baking sheets, spacing them about 3 inches apart.

Bake for 15 minutes, then remove the pans from the oven.

Gently flatten the cookies with a spatula.

Rotate the pans from top to bottom and front to back and continue baking until the cookies are lightly cracked on the top and golden brown on the bottom, about 25 minutes longer.

Let cool slightly, then transfer to racks to cool completely.