Place the water into a pan (roughtly a 1L milk pan). Add the sugar carefully aiming not to splash it up the sides of the pan. Set the pan over a low to moderate heat to allow the sugar to dissolve into the water. Once dissolved, increase the heat under the pan a little. Without stirring allow the sugar to caramelise whilst the water evaporates. Once it is a moderate brown colour (similar to the brown of a corrugated cardboard boadd the cream. Be aware that the mixture will react to the cold of the cream by rising up the pan. Stir. Continue stirring until the caramel has reached 104℃ / 219℉ (see note b below). Pour the caramel into a bowl and set too cool.

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.

Pour the cream into a small pan and set over a medium heat. Allow the cream to heat through but avoid letting it boil. Meanwhile break the chocolate into small pieces. Add the chocolate pieces to the hot cream. Use a teaspoon to stir the ganache, gradually the chocolate will melt and the ganache will become smooth.

Pour the warm chocolate ganache over the set fudge. Use the back of a spoon to level it off. Scatter over sprinkles (optional). Set aside to firm up.

Once fully cold and firm use the excess greaseproof paper as handles to remove the fudge from the tin. Use a sharp knife to slice the fudge into bite sized pieces.Enjoy!