Lightly grease and fully line the brownie tin. Ensure that the greaseproof paper goes beyond the rim of the pan, this will help to remove the fudge once it has firmed up.
Place the double cream, milk, butter and sugars into a large heavy based pan. Place over a low heat to allow the ingredients to melt. Constantly stir using a wooden spoon. Ensure all of the ingredients have completely melted before moving onto the next stage - you should no longer hear or feel the granulation of the sugar. Pay particular attention to the 'corner' of the pan when listening for sugar granulation.
Set the digital thermometer to 117C /242F and attach it to the side of the pan. Ensure that the thermometer is secure and not touching the base of the pan. (Alternatively use a thermospatula which has the thermometer built into the spatula, or see note f below).
Increase the heat under the pan to allow it to come to the boil whilst stirring all of the time. Boil the mixture, whilst continuing to stir, until it reaches 117C / 242Please be careful - the mixture is very hot! You may find it helpful to wear an oven mitt in case of splashes from the sugar.
Once the required temperature has been reached, remove the pan from the heat and leave it undisturbed to cool down to 110C / 230This will only take a couple of minutes.
Beat the mixture with the wooden spoon vigorously until it is no longer shiny.
Pour the half of the fudge mixture into the prepared tin. Smooth it into the corners. Spoon blobs of the cooled caramel onto the fudge. Use a knife to marble it into the fudge (see note e below). Top with the remaining fudge. Spread the top layer of fudge out with the back of a spoon. Add a few more drops of caramel and use the knife to ripple into the fudge. Smooth the surface of the fudge with the back of a spoon as best you can (it will be getting thicker at this stage).
Set aside to cool at room temperature for at least three hours to firm up.