Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans. In a large bowl, whisk together flour, cocoa powder, baking powder, and salt.

In another bowl, mix eggs, milk, and oil until combined. Gradually combine wet and dry ingredients, stirring just until smooth.

Divide the batter evenly into the prepared pans and bake for 25-30 minutes. Check doneness with a toothpick; it should come out clean.

Allow the cakes to cool in the pans for 10 minutes before transferring to a wire rack.

While the cakes cool, prepare the pistachio mousse. In a chilled mixing bowl, beat heavy cream with powdered sugar until stiff peaks form, approximately 3-5 minutes.

Gently fold in the pistachio paste until the mixture is well incorporated and velvety in texture.

Keep the mousse in the refrigerator to maintain its fluffiness while you assemble the cake.

Once the chocolate cakes are completely cooled, place one chocolate layer on a serving plate and spread a layer of pistachio mousse on top.

Place the second layer of chocolate cake on top and spread the mousse evenly on top and sides.

Use a spatula to ensure the mousse covers all visible layers beautifully.

Heat the heavy cream in a saucepan over medium heat until it begins to simmer.

Remove from heat and pour the hot cream over the chopped semi-sweet chocolate in a bowl.

Allow it to sit for a couple of minutes, then stir vigorously until smooth and glossy.

Let the ganache cool slightly before pouring it over the assembled cake.

Refrigerate the entire layered cake for at least 2 hours to allow it to set properly.

Garnish generously with chopped pistachios and chocolate shavings before serving.