Shrimp and Herb Canapés

Make the herb butter: Combine watercress, parsley, tarragon, and ½ cup water in a food processor and purée into a smooth paste; transfer to a 2-qt. saucepan, and heat over medium heat. Cook, without stirring, until mixture barely begins to simmer; remove from heat and let sit for 10 minutes. Using a slotted spoon, scoop the herb solids from the liquid and transfer to paper towels to drain. Squeeze paper towels around herbs to remove any excess liquid, and then transfer herb solids to a food processor along with 2 sticks butter and 2 tbsp. lemon juice; process until smooth. Transfer to a bowl, season with salt and pepper, and set herb butter aside.

Make the shrimp butter: Bring a 2-qt. saucepan of water to a boil, and add unpeeled shrimp; cook until just cooked through, about 4 minutes. Drain, peel, devein, and finely chop; transfer to a food processor along with remaining butter and lemon juice, and process until smooth. Transfer to a bowl, season with salt and pepper, and set aside.

Thinly spread some of the herb butter over half the bread slices; thinly spread some of the shrimp butter over the remaining bread slices. Transfer each butter to a separate piping bag fitted with a ⅜″ star tip, and pipe rows of butter along the long edges of the corresponding herb and shrimp buttered bread slices; chill to firm butters. Cut each bread slice crosswise into 5 rectangles. Garnish rectangles with capers, cooked shrimp slices, radishes, and chives.