My guess would be the scene, where Kitty and Levin open their hearts to each other. It fits the description (tr. Constance Garnett).
“Ah! I’ve scribbled all over the table!” she said, and, laying down the chalk, she made a movement as though to get up.
“What! shall I be left alone—without her?” he thought with horror, and he took the chalk. “Wait a minute,” he said, sitting down to the table. “I’ve long wanted to ask you one thing.”
He looked straight into her caressing, though frightened eyes.
“Please, ask it.”
“Here,” he said; and he wrote the initial letters, w, y, t, m, i, c, n, b, d, t, m, n, o, t. These letters meant, “When you told me it could never be, did that mean never, or then?” There seemed no likelihood that she could make out this complicated sentence; but he looked at her as though his life depended on her understanding the words. She glanced at him seriously, then leaned her puckered brow on her hands and began to read. Once or twice she stole a look at him, as though asking him, “Is it what I think?”
“I understand,” she said, flushing a little.
“What is this word?” he said, pointing to the n that stood for never.
“It means never,” she said; “but that’s not true!”
He quickly rubbed out what he had written, gave her the chalk, and stood up. She wrote, t, i, c, n, a, d.
Dolly was completely comforted in the depression caused by her conversation with Alexey Alexandrovitch when she caught sight of the two figures: Kitty with the chalk in her hand, with a shy and happy smile looking upwards at Levin, and his handsome figure bending over the table with glowing eyes fastened one minute on the table and the next on her. He was suddenly radiant: he had understood. It meant, “Then I could not answer differently.”
He glanced at her questioningly, timidly.
“Yes,” her smile answered.
“And n... and now?” he asked.
“Well, read this. I’ll tell you what I should like—should like so much!” she wrote the initial letters, i, y, c, f, a, f, w, h. This meant, “If you could forget and forgive what happened.”
He snatched the chalk with nervous, trembling fingers, and breaking it, wrote the initial letters of the following phrase, “I have nothing to forget and to forgive; I have never ceased to love you.”
She glanced at him with a smile that did not waver.
“I understand,” she said in a whisper.
He sat down and wrote a long phrase. She understood it all, and without asking him, “Is it this?” took the chalk and at once answered.
For a long while he could not understand what she had written, and often looked into her eyes. He was stupefied with happiness. He could not supply the word she had meant; but in her charming eyes, beaming with happiness, he saw all he needed to know. And he wrote three letters. But he had hardly finished writing when she read them over her arm, and herself finished and wrote the answer, “Yes.”
“You’re playing secrétaire?” said the old prince. “But we must really be getting along if you want to be in time at the theater.”