Preheat the oven to 350 degrees. Butter and flour a 9inch springform pan. In a large mixing bowl cream the butter with an electric mixer until soft. Add the sugar and beat until well blended. Add the eggs and vanilla and beat until light and very fluffy.
Thoroughly combine the flour, ground oats, baking powder, cardamom, and salt in a medium bowl. Beat these ingredients into the butter mixture, alternating with the milk, until well blended. Scrape into the prepared pan and smooth the top. Bake 50 to 55 minutes, or until a knife inserted in the center of the cake comes out clean. Do not overcook. Cool on a wire rack 15 minutes, then loosen the edges of the cake, invert onto a platter, and invert again onto the rack so the cake is right side up. Let the cake cool completely, about 2 hours.
When the cake is cool, make the frosting: In a small saucepan melt the butter over medium low heat. Add the brown sugar and stir until melted and blended with the butter. Slowly pour in the milk, mix well, and bring the mixture to a boil. Scrape it into a medium bowl and cool 10 minutes. Add the confectioners sugar and vanilla and beat with an electric mixer until smooth. Spread all over the cooled cake. Decorate the cake by sprinkling the ground nuts on top, leaving a 3/4-inch border bare. Wait at least 15 minutes before serving to allow the frosting to set