Prep the pan and preheat the oven. Remove from the heat, stir in vanilla extract and salt (to taste), and mix thoroughly to combine. Pour into a bowl and set aside to cool for 10 minutes. Preheat oven to 180°C (350°F). Grease with butter and line a 2lb 23cmx14cm (9x5.5inch) baking pan with parchment paper. Leave an overhang on the sides so that it makes it easier to lift the brownies out afterwards.
Freeze caramel. Cut yourself a piece of parchment paper and lightly grease it. Dollop the caramel onto the paper and smooth it out with the back of the spoon. You don't want to make it too thin. Place the paper in the fridge for 30 minutes minimum. Longer if possible.
Melt butter and chocolate. Set a bowl over a saucepan of simmering water, making sure not to let the water touch the bottom of the bowl. Place the chocolate and butter into the bowl and stir until melted. Alternatively, place in a heatproof bowl and microwave for 10-second intervals stirring between bursts until melted.
Add the sugar. With the bowl still set over the water, add the white and light brown sugar to the chocolate and stir for a minute
Add eggs. Add the egg, yolk and vanilla extract into the chocolate mixture and using hand-held electric beaters, beat n medium to high for 3 to 4 minutes until thickened and aerated. When the beaters are lifted out of the mixture the mixture should form a ribbon on the surface.
Sift in the dry ingredients. Sift into the bowl the flour, cocoa powder and salt and fold through until a few streaks remain. This makes sure that you won't overmix it.
Add brownie batter and caramel to the pan and bake. Pour half the batter into the prepared loaf pan and smooth flat with an offset spatula. Remove the parchment paper with the caramel from the freezer and peel off the paper. Lay the frozen caramel onto the batter. (If using unfrozen caramel then drizzle the caramel over the batter.) Spoon the remaining batter over the top of the caramel and smooth with an offset spatula. Bake brownies for 22-25 minutes until set around the edges but jiggly in the centre. Remove from the oven and cool in the pan for 30 minutes. Use the parchment paper sling to gently lift the brownie out of the pan and onto a wire rack to cool before cutting. Drizzle with remaining caramel over the brownies and sprinkle with flaked sea salt. (If the caramel is too stiff to pour, then microwave to heat it slightly to loosen it). Serve warm or refrigerate for 1 hour to get clean slices when cut.