Espresso - Milk - Guatemala
Unpleasant acidity (what a lot of people refer to as sourness) in an espresso usually comes as a result of either the following (1) a green coffee with a high natural acidity, (2) a brew ratio that doesn't fit with the style of roast you brew with or (3) a lighter roasted coffee.
Use a 1:2 brew ratio. With all espresso you first need to identify the size of the filter basket - preferably of the brand VST ridgles - that you are using. For this coffee that would be the filter basket size * 2, aiming for a target TDS of 9,5. Depending on which grinder and set-up your working with the result will be a bit different but the recommended brew time is still between 30 - 32 seconds for this coffee.
The brewing always effects how the coffee will taste. And with a good combination of roasting and brewing a milk coffee doesn't have to be dark roasted to taste rich and sweet. This is an great example of that.