Preheat your oven to 350°F (175°C).
In a mixing bowl, cream together the peanut butter, sugar, and softened butter until smooth.
Add the eggs one at a time, beating well after each addition.
Gradually add the milk, mixing until well combined.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Pour the batter into a greased baking pan.
Bake for 30 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.