If you want a writing prompt maybe kirk discovering a cluster of freckles on the nape of spock's neck?
ah hopefully this fit your prompt well enough!!
Constellations (find it on ao3 here)
The dark silhouette remained still, framed in the doorway to the fresher. Jim blinked, then reached out a hand, making his voice softer, more honeyed.
“Spock. Come on, sweetheart. Come to bed with me. Please.”
Spock finally moved, walking towards Jim and settling on the edge of the bed, keeping his face turned away. He took off his socks but made no move to remove the Vulcan robes he had been wearing during meditation. Jim frowned a bit, catching the end of one of the trailing sleeves. A few weeks into their relationship, Spock had discovered just how comfortable Jim’s old loose t-shirts were, and refused to wear anything else to bed. For Spock to not take off his robes to sleep… well, it certainly planted worry in Jim’s head. “Aren’t you gonna take this off?” He asked, keeping his voice soft in the dim light.