In the mixer bowl, combine the flour, sugar, baking soda, & salt - set aside.
In a small saucepan, combine the butter, peanut butter, and water.
Heat over medium and bring to a boil.
Pour the butter mixture over the dry ingredients & beat to combine. The mixture will cool slightly as you are beating.
Add in the milk, eggs, and vanilla and continue beating until combined.
Pour the batter onto the prepared baking sheet and bake for approx 20 min or until toothpick test is clean.
While it's baking prepare your frosting
Frosting
Combine the butter and cream cheese in a mixer bowl & beat well until smooth.
Slowly beat in the confectioner's sugar, brown sugar, and maple syrup until the sugar has blended and dissolved into a creamy mixture. You can add a splash of milk if needed.
Once the cake is finished baking and cooling - spread the frosting over the top of the cake with an offset spatula.