Cucumber and Smoked Salmon Mousse Sandwiches

Place the smoked salmon in a food processor fitted with the metal blade or a blender and process until relatively smooth, 45-60 seconds, stopping the machine once or twice to scrape down the sides of the bowl. Add the mayonnaise and sour cream and continue to blend until the mixture is as smooth as possible, 30-45 seconds longer, adding the chopped dill and pepper during the final few seconds, again stopping the machine as necessary. Transfer to a bowl and nest it inside a larger bowl filled with ice. Cover and refrigerate.

Place the cream in a chilled bowl and whisk until soft peaks form. Remove the salmon mixture from the refrigerator and, using a rubber spatula, gently fold in the whipped cream. Do not overfold or the mixture will be grainy and will lose its shape. It should be firm enough to hold its shape when spooned out. Nest the bowl in the ice again and refrigerate. Preheat a broiler.

Using a pastry brush, lightly brush both sides of each bread slice with the melted butter. Arrange the bread slices on a baking sheet and place under the broiler. Toast, turning once, until golden brown on both sides, about 30 seconds on each side. Remove from the broiler. Using a round cutter the same diameter as the cucumber, cut out 4 rounds from each bread slice.

TO ASSEMBLE: Cut the cucumber crosswise into 48 thin slices. Place the rounds of toasted bread on a work surface and put a small spot of the chilled salmon mousse on each--just enough to hold a cucumber slice in place on the bread. Now place a slice of the cucumber on each bread slice and press it gently. Spoon about 1 tablespoon of the mousse onto the center of the cucumber slice, top with another cucumber slice, then repeat the process again so that there are 3 cucumber slices sandwiching 2 layers of mousse. Repeat this for the remaining toasts.

Decorate each sandwich with a small dollop of the mousse and a dill sprig. Transfer to a platter and serve immediately.