Preheat oven to 300°F. Line two large baking sheets with parchment paper; set aside.
In a large mixing bowl, beat the butter with the sugar and salt until light and fluffy. Combine the vanilla with the baking soda and add to the butter/sugar mixture. Stir in the flour 1 cup at a time. Gently stir in the Rice Krispies. Do not over mix.
Scoop 2-tablepoon sized mounds onto prepared baking sheets, making sure to space them at least 2 inches apart. Bake 15 minutes. Remove from oven and quickly but gently flatten with a spatula. Return to oven (rotating pans front to back, top to bottom) and bake an additional 25 minutes, until lightly golden and cracked on top.
Remove from oven and let cool on baking sheet 8 minutes before transferring to wire rack to cool completely.