Marinate chicken with white pepper, soy sauce, garlic paste and salt. Let it sit for 10 minutes.
Heat oil in a frying pan, and add the marinated chicken.
Cook for about 10 - 15 minutes until the chicken is tender, and the water has dried.
Remove the chicken and let it cool.
Add the chicken in a food processor with 4 tablespoons mayonnaise and the evaporated milk. Pulse till the chicken shreds and turns into a paste. If the chicken spread feels too thick, add another tablespoon of mayonnaise.
The consistency of the chicken spread will depend on how much you pulse it. If you pulse more, the spread will be more like a paste but if you pulse less, the chicken spread will have more texture. So process as much as you prefer.
Take out the chicken spread and place it in a bowl. Taste and adjust seasonings if required.
To make the sandwich, spread the chicken spread on one bread slice, and then place 4 cucumber slices on top. Top with the other bread slice, and then cut into half or triangles.