From YA Muses:
Break-Down of a "Melty Romance"
1. Character chemistry develops naturally.…Character chemistry is related to pacing. A melty romance has a natural pace to it – too fast, and the reader might question the chemistry; Too slow, and that emotional resonance might just not be there.
2. "Forbidden Fruit" and "Opposite Worlds" romances build tension.… In your book, consider the external factors at play, and see how you can dial up what's at stake for the romance to boost reader interest, grow the characters' emotional bond with one another, and make it meltier than ever.
3. Love triangles are popular – but not always necessary.Romantic plot lines can have a love triangle, but it's not necessary to make it resonate as sexy & melty with the reader.
4. Romantic chemistry needs to be off-the-charts.…When a romantic connection builds in a natural way, and the characters' emotional and physical connection is unstoppably strong it just leaps from the page, that's melty.
Link to Writing the “Melty Romance” by Jodi Kendall at YA Muses.