Place the 8 large eggs in a saucepan, cover generously with cold water and bring to a boil over medium-high heat. Once the water is boiling, lower the heat slightly and cook for 9–10 minutes, until the yolks are fully set. While they cook, fill a large bowl with ice and cold water. Transfer the eggs straight into the ice water and leave them until completely cold, then peel carefully. The yolks should be a genuinely clean, bright yellow right through, never chalky or ringed with grey. That immediate chilling is what stops the cooking at the right moment, so the whites stay tender and the yolks keep their colour.

Separate the peeled eggs into whites and yolks. Mash the yolks finely in a bowl with a fork until powdery and smooth, then chop the whites into small but visible pieces and fold them back in. Add 120–140g of Japanese mayonnaise, 4–5g of fine salt, a small pinch of pepper and, if using, 4–6g of sugar and 10–15g of softened butter, and mix until creamy and cohesive. Only if the mixture seems stiff, loosen it with 1 tbsp of milk or cream, adding a second only if needed. Taste before assembling, since Japanese egg sandwiches are gently seasoned rather than aggressively salty. That balance of fine yolk and chunky white is what gives the filling richness without turning it into paste.

Lay out the 8 slices of milk bread and trim the crusts cleanly from all four sides, so every slice is an even, tidy rectangle. Spread each slice lightly with the 20–25g of softened butter, about 2½–3g per slice, right to the edges. If using karashi, spread an extremely thin layer on the bottom slice of each sandwich only. Use genuinely soft, fresh bread that feels fluffy and almost cloud-like. That thin butter coating is what acts as a moisture barrier, so the bread stays soft and light instead of going damp under the filling.

Divide the egg filling evenly into 4 portions of about 130–140g each, then spread or mound one portion generously over each of 4 buttered slices, taking it all the way to the edges. Top with the remaining 4 slices, buttered side down, and press very gently so the filling settles evenly right to the corners. Keep the layers level so the cut faces look neat and full. That light pressure is what lets the filling genuinely fill the sandwich without squashing the crumb flat.

For the cleanest slices, wrap the sandwiches lightly or set them seam-side down and chill for 5–10 minutes. Using a very sharp knife, cut each sandwich cleanly in half into 2 rectangular halves, wiping the blade between cuts if filling clings to it. Arrange the halves upright or slightly leaned so the creamy cross-sections show. The bread edges should stay clean and tidy. That brief chill is what firms the filling just enough to hold its shape under the blade, giving genuinely sharp, neat edges.