Brown the ground beef and finely diced onion in a large skillet or saucepan over medium heat, breaking the beef into small pieces. Cook for about 6 to 8 minutes, or until no pink remains.

Drain any excess grease from the skillet. Return the beef mixture to the pan if necessary.

Stir in the canned chili, undrained RO*TEL, and optional chili powder until the mixture is evenly combined.

Reduce the heat to low and add the cubed Velveeta along with the milk. Stir frequently for about 5 to 8 minutes, until the cheese has completely melted and the dip is smooth; do not let it boil.

Transfer the hot dip to a serving bowl or small slow cooker. If it thickens while sitting, stir in a small splash of milk.

Spoon the sour cream into a small plastic bag or piping bag and snip off a tiny corner. Pipe concentric circles over the dip, then drag a toothpick or skewer from the center outward at intervals to form a spiderweb; add sliced black olives as a simple spider if desired.

Serve the dip hot with tortilla chips. Keep it warm during serving and stir occasionally for the creamiest texture.