Cream the butter and brown sugar together in a bowl until light and fluffy, using an electric mixer at medium speed. Add the eggs, one at a time, beating well after each addition. Add the cooled chocolate, coffee powder, and rum flavoring, blending well. Stir in the flour and the chopped walnuts and turn into the unbaked pie shell. Arrange the walnut halves in a circle around the edge of the pie. Bake in a preheated 190ºC . oven for 25 minutes or until set around the edge. Cool on a wire rack.