Lightly butter an 8x8-inch pan or line it with parchment, pressing firmly into corners to prevent leaks.

In a large microwave-safe bowl, stir together melted butter, corn syrup, both sugars, and sweetened condensed milk until smooth.

Microwave on high for 6 minutes, pausing to stir at the 2-minute and 4-minute marks. Mixture will bubble—use a large bowl to avoid overflow.

Caramel should reach soft-ball stage (240°F). For firmer texture, microwave 30–60 seconds more, watching closely.

Carefully stir in vanilla and salt (if using). Use a long-handled spoon; the caramel will be very hot.

Pour into the prepared pan, smooth the top, and let cool at room temp for 1–2 hours.

Once firm, cut into pieces and wrap individually in wax paper if desired.