Heat oven to 375 degrees F. Lightly grease or spray a 9- by 9-inch pan.
Combine flour, 1/2 cup each sugar and brown sugar, baking powder and salt in a medium bowl. Add milk and butter and whisk until smooth. Stir in the rhubarb. Pour into the prepared pan.
Combine the remaining 1 tbsp. sugar, cinnamon and cardamom in a small dish. Sprinkle over the batter.
Bake 30-40 minutes or until a wooden pick inserted in the center comes out clean. Cool slightly on a wire rack.
For the sauce, combine the sugar, butter and whipping cream in a small saucepan. Bring to a boil over medium high heat, stirring constantly. Reduce the heat to low and simmer 5 minutes or until slightly thickened, stirring occasionally. Stir in the lemon juice. Serve the sauce warm or refrigerate until needed.