Pack the brown sugar into the measuring cup and level it. Measure the cream, cube the butter and set out the salt and vanilla. Use a heavy-bottomed saucepan with at least a 2-quart capacity.

Put brown sugar, butter, cream and salt into the saucepan. Warm over medium-low heat, stirring with a heatproof spatula until the butter melts and the mixture becomes uniform. Scrape the bottom gently so sugar does not stick.

When small bubbles appear, reduce the heat to maintain a gentle simmer. Cook for 3–5 minutes, stirring frequently, until the mixture looks smooth and slightly thicker. Do not cook to a dark color or a candy stage; it will thicken further as it cools.

Take the saucepan off the heat and stir in the vanilla carefully. Hot sugar can cause serious burns; do not touch or taste the dip while it is hot.

Transfer carefully to a clean heatproof bowl. Let cool for about 20 minutes, stirring occasionally, until comfortably warm and thick enough to coat a spoon. Cooling time varies. If it is still hot, wait longer before serving. Refrigerate promptly if making ahead.

Wash apples under running water, dry them, core and slice shortly before serving. Portion dip into a small clean bowl and serve with apple slices. Keep the remaining batch refrigerated; do not return used dipping sauce to it.