Heat oven to 400°. Place bacon on a rimmed baking sheet fitted with a wire rack. Bake at 400° for 20 to 25 minutes, until crispy.
Fill a large, deep-sided skillet threefourths full with water. Bring water to barely simmering and cover. Meanwhile, in a small pot, whisk buttermilk, egg yolk, cornstarch, 1/4 tsp of the salt and the cayenne over medium heat until smooth. Cook, stirring constantly, until liquid barely simmers and is thickened, about 3 minutes, being careful not to scramble eggs. Remove from heat and stir in lemon juice, chives and butter. Cover with lid to keep warm.
Sprinkle tomatoes with remaining 1/8 tsp salt. Place 2 slices toasted bread on each plate. Top with some sliced tomatoes, 1/2 cup of the arugula and 2 slices bacon (each slice broken in half). Crack 4 of the eggs into separate cups. Pour into skillet of simmering water one by one. Poach eggs 3 to 4 minutes, until whites are set. Remove with a slotted spoon and place one on each slice. Repeat with remaining 4 eggs. Spoon sauce over eggs.