Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large bowl, cream together the sugar and peanut butter until smooth. Beat in eggs, one at a time, then stir in the vanilla.
In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add to the peanut butter mixture, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.
Pour batter into the prepared pan and bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
Let cool completely before frosting.
Notes
This cake is best served at room temperature. You can also add chocolate chips or other mix-ins to the batter.