Preheat your oven to 350°F (175°C). Grease and flour a 10-inch bundt pan to ensure the cake doesn't stick.
In a large mixing bowl, combine the all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt. Whisk them together until well combined.
Create a well in the center of the dry ingredients and add the milk, vegetable oil, eggs, and vanilla extract. Using an electric mixer, beat the mixture on medium speed until well combined and smooth.
Gradually add the hot water to the batter, mixing on low speed. Be careful not to overmix. The batter will be thin, but that's what will give the cake its moist and tender texture.
Pour half of the batter into the prepared bundt pan, spreading it evenly. Using a spoon, dollop the peanut butter onto the batter, making sure to leave a space around the edges. Swirl the peanut butter gently into the batter using a butter knife or a skewer.
Pour the remaining batter over the peanut butter layer, ensuring it covers the peanut butter completely. Smooth the top with a spatula.
Place the bundt pan in the preheated oven and bake for approximately 45-50 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, carefully invert the pan onto a wire rack to release the cake. Allow it to cool completely before serving.