1.Heat oven to 350 degrees . Grease and flour Canoe pan; set aside.
2. Combine flour, baking powder and salt in a small bowl. Whisk to blend. Heat 1/2 cup water and the butter in a small sauce pan until the water just boils and the butter melts. Remove from heat.
3. Beat the eggs with an electric mixer on high for 2 minutes. Gradually add the sugar and beat until thick and golden, 4 to 5 minutes longer. Fold in the flour mixture until just blended. Add the water and melted butter and stir until just blended.
4. Divide half of the batter evenly among the prepared pan indents, a scant 1/4 cup per cake.
5. Bake until cakes are golden and a toothpick inserted in centers comes out clean, about 15 to 17 minutes. Invert cakes onto a wire rack to cool. Wash pan and repeat with the remaining batter. Transfer to a wire rack and cool completely.
Filling:
1. Beat butter in a medium-size bowl until light and fluffy. Gradually add confectioners sugar. Add 1 tablespoon of the milk and beat 5 minutes until almost white and light and fluffy. Add extra milk if filling is too stiff. Spoon frosting into a pastry bag fitted with a small round tip (or use the decorating set supplied with pan). Remove cakes from pan. Insert tip of the pastry bag into the center of the flat side of the cakes and squeeze. Repeat on either side for a total of 3 spots of frosting piped into the base of each cake, resembling the classic cream-filled treats. Flip over, so cake sits flat-side down. Repeat with all cakes and filling and serve.