Canadian barista champion Ben Put of Phil & Sebastian writes about how they recently dialled in their espresso shots when pulling for drinks which include milk:
Espresso can be a very intense and acidic drink if it is not properly brewed. In order to create a balanced espresso, baristas will often adjust how much blonde they add to the end of the shot. The blonde is low in acid and does not contain much coffee solubles so it does a very good job balancing the espresso. Generally, the espresso parameters used to dial in are designed to create this style of espresso: straight espresso for drinking.
Lattes and cappuccinos present a different acidity/bitterness ratio because they contain a third ingredient: milk. Because the milk is sweet and is not acidic it does a very good job softening the intensity and acidity of the espresso. Now a smaller, less extracted, more intense shot is a good thing. The lower amount of extraction will ensure that there is not any perceivable bitterness in the latte and the lower amount of total water in the espresso means that the milk isn’t getting too watered down, which maximizes the sweetness of the drink.