Apr 9, 2022 · How to Make Peanut Butter Cake. 1. In a small saucepan, bring water, butter, and peanut butter to a simmer over medium heat, constantly whisking, until melted and smooth, about 3 minutes. 2. Whisk together the flour, sugar, baking soda, and salt in a large bowl. 3. Add the milk, eggs, and vanilla.