Preheat your oven to 350°F (175°C) and spread the hazelnuts in a single layer on a baking sheet. Bake them for about 10 minutes, or until they are golden brown and aromatic. Allow to cool slightly and roughly chop.

In a medium-sized bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and ground cardamom. Set aside.

In a large mixing bowl, beat the softened butter and sugar together on medium speed until light and fluffy, about 3-5 minutes. Gradually add eggs, one at a time, mixing well after each addition, then stir in vanilla extract.

With the mixer on low speed, gradually add half of the dry mixture to the butter mixture, followed by the milk, mixing until just combined. Add the remaining dry ingredients, mixing gently until the batter is smooth. Fold in most of the chopped hazelnuts.

Grease a 9x9-inch baking pan and pour the batter into the pan, smoothing the top. Bake for 25-30 minutes, or until a toothpick comes out clean. Allow to cool in the pan for 10 minutes before transferring to a wire rack.

In a mixing bowl, beat the softened butter until creamy. Gradually add powdered sugar and cocoa powder, mixing well. Add milk or cream until the frosting reaches your desired consistency.

Once the cake is completely cooled, spread the chocolate frosting over the top and sprinkle the reserved chopped hazelnuts on top.