1. For hollandaise sauce, melt butter in a small saucepan over medium heat; cool. In a blender, place egg yolks, champagne vinegar, lemon juice and ½ teaspoon salt. Cover and blend on high for 2 minutes. Reduce speed to low. With blender running, slowly add cooled melted butter. Continue blending for 2 to 3 minutes more or until slightly thickened.* Season to taste with additional salt.
2. To poach eggs, fill a 5- to 6-quart Dutch oven with 3 quarts water; bring to a gentle simmer. Stir in white vinegar. Break 4 eggs each into a separate shallow cup, being careful not to break the yolks. Using a large spoon, stir water mixture. While water is still moving, slide each egg into the simmering water. Poach eggs at least 3 minutes, turning occasionally, or until the whites are completely set and the yolks reach desired doneness. Carefully remove eggs from water with a slotted spoon. Drain on paper towels; repeat poaching with remaining 4 eggs.
3. To assemble servings, split each biscuit. Place biscuit bottoms, cut sides up, on serving plates. Top with cucumbers, salmon, poached egg and hollandaise sauce. Garnish with radish slices and microgreens, if desired. Serve immediately.

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*Note: If hollandaise sauce separates, transfer sauce to a glass measuring cup. Do not wash blender container; add 1 tablespoon boiling water. Cover and blend on high for 30 seconds. Reduce speed to medium. With blender running, slowly add sauce. Blend for 30 to 60 seconds or to desired thickness.