The Waffle Longsleeve in Black from Purple Mountain Observatory, UK. 

Details
Longsleeve Waffle crew with sweat-wicking and quick-drying capabilities while remaining lightweight and breathable, making it a summer essential. A heat-sealed front pocket with a power mesh lining, whilst it’s hidden YKK® magnetic zip keeps your essentials secure without adding bulk.

• Sweat-wicking & quick-drying properties
• A heat-sealed front pocket with a power mesh lining