Preheat oven to 180°C / 350°F. Spray or butter 11 inch tart/flan pan.
In a medium bowl, whisk together cake mix, ground pistachio and cardamom.
In a large bowl, beat the dry ingredients, milk, oil and eggs with electric mixer on low speed 30 seconds, then on medium speed 2 minute. Pour batter in prepared pan.
Bake 22 to 28 minutes or until toothpick inserted in center comes out clean. Cool in pan on wire rack 10 minutes. Turn out onto a serving plate and cool completely.
While the cake is cooling, make the muhalabia.
In a medium saucepan, combine the milk and cornstarch until the cornstarch is dissolved and heat over medium heat and keep whisking until it becomes thick. Remove the pan from the heat and pour it over the cake.
Decorate it with dried roses and pistachio. Refrigerate it for about 30 minutes or until the muhalabia is settled.