Preheat the oven to 350 degrees F. Spread the 1/8 cup butter into the bottom of a 9-inch, non-stick cake pan. Combine the 1/3 cup brown sugar and spices in a bowl. Sprinkle mixture into the pan. Top brown sugar with rhubarb.
Combine flour, baking powder and salt in a medium bowl. Place ½ cup butter, ¼ cup brown sugar, granulated sugar and vanilla in a large bowl and beat with an electric mixer until combined and lightened. Beat in the eggs, one at a time. Gradually beat in the flour mixture, alternately with the buttermilk.
Spoon and carefully spread the batter over the rhubarb. Bake 45 minutes, or until the cake springs back when gently touched in the centre. Cool the cake 15 minutes. Run a paring knife around the edge of the cake. Set a cake plate over the pan. Carefully and quickly invert the cake and release onto the plate.
If desired, served the cake with unsweetened whipped cream, or vanilla ice cream.
Note: The rhubarb should be cut into ½-inch pieces. Before using frozen rhubarb, thaw it, drain away excess liquid, and pat dry with paper towel.