Preheat your oven to 350°F (175°C). Grease the 9x13 inch baking pan.
In a large mixing bowl, combine the flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt. Mix well.
Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Mix until combined.
Slowly add the boiling water to the batter, mixing gently. The batter will be thin; this is normal.
Pour the batter into the prepared baking pan and bake for 30 minutes or until a toothpick inserted into the center comes out clean.
Once baked, remove the cake from the oven and let it cool for about 15 minutes in the pan.
Using a toothpick or skewer, poke holes all over the surface of the cake.
In a separate bowl, mix peanut butter and powdered sugar until smooth. Spread this mixture over the cake, making sure it fills the holes.
In another bowl, whip the heavy cream and 1 teaspoon of vanilla extract until soft peaks form. Spread this over the cake to cover the peanut butter layer.
Optional: Garnish with chocolate shavings or candies.
Refrigerate the cake for at least 2 hours before serving to allow the flavors to meld.