Preheat oven to 180 C / Gas 4. Grease and flour a 20x30cm (8x12 in) cake tin.
For the cake, sift together the flour, baking powder, mixed spice and nutmeg. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the milk. Spread cake mixture into prepared tin. Sprinkle the chopped rhubarb over the mixture.
For the topping, mix together the dark brown soft sugar, mixed spice, nutmeg and almonds. Sprinkle over top of the cake in the tin.
Bake in the preheated oven for 50 to 60 minutes, or until the cake springs back when lightly tapped. Serve warm or cold.