Preheat oven to 180º C.
In a small bowl, mix the melted butter and milk.
In a separate medium-sized bowl, beat the eggs and sugar together until light and fluffy.
While continuing to stir, slowly pour the melted butter mixture.
Add flour, cardamom and baking powder, and mix until well combined.
Pour batter into an 8-inch springform pan, and sprinkle the diced rhubarb over the top, making sure the rhubarb DOES NOT touch the sides of the pan.
Bake for about 45 minutes to 1 hour, until the cake teste pick comes out clean.
Allow for the cake to cool and carefully remove from the pan.
Serve with whipped cream or a scoop of vanilla or strawberry ice cream.