Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
In a separate bowl, whisk together the eggs, milk, oil, and vanilla extract.
Add the wet ingredients to the dry ingredients and mix until just combined. Gradually stir in the boiling water until the batter is smooth. The batter will be thin.
Pour the batter evenly into the prepared cake pans.
Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.
Make the Whipped Cream Filling
In a chilled bowl, beat the heavy whipping cream with an electric mixer until soft peaks form.
Gradually add the powdered sugar and vanilla extract, and continue beating until stiff peaks form.
Prepare the Chocolate Frosting
In a large bowl, beat the softened butter until creamy.
Add the cocoa powder and mix until well combined.
Gradually add the powdered sugar, alternating with the milk, and beat until smooth and spreadable. Stir in the vanilla extract.
Assemble the Cake
Place one cooled cake layer on a serving plate. Spread an even layer of the whipped cream filling over the cake.
Top with the second cake layer.
Frost the entire cake with the chocolate frosting. Decorate as desired.
Notes
For best results, ensure all ingredients for the whipped cream are very cold. Chill the cake before slicing for cleaner cuts.