01 -
Line a 9x13-inch pan with parchment paper, leaving overhang on two sides. Lightly butter or oil the parchment.
02 -
Combine granulated sugar, brown sugar, corn syrup, butter, and fine salt in a heavy 4 to 5-quart saucepan. Cook over medium heat, stirring, until melted and bubbling gently, about 5 minutes.
03 -
Slowly pour in the room-temperature cream while whisking constantly. The mixture will bubble up, so keep stirring until it looks smooth.
04 -
Attach a candy thermometer and boil over medium heat, stirring often, until the caramel reaches 245°F, about 20-25 minutes. Keep the thermometer probe in the caramel and off the pan bottom.
05 -
Remove the pot from the heat and carefully stir in the vanilla. Pour the caramel into the prepared pan and smooth the top gently. Sprinkle with flaky salt after 30 minutes if using.
06 -
Let the caramel cool at room temperature for at least 4 hours. Lift out the slab, cut into small squares with a lightly oiled knife, and wrap each piece in wax paper.