Line an 8x8 inch square baking tin with baking paper.
To make the lemon simple syrup, use a peeler to peel just the yellow part of one lemon (you don't want the white part).
Juice the lemons until you have 1/2 cup of strained lemon juice (make sure it's free of pulp and pips).
Place the peel from one lemon, lemon juice and sugar in a small heavy-based saucepan over medium heat.
Bring it to a boil and cook stirring every so often for 7-10 minutes, until thickened and reduced by about half. It should be the colour of very pale honey. Set aside.
Combine the sweetened condensed milk, butter and white chocolate in a medium heavy-based saucepan. Stir over low-medium heat, without boiling, until everything is melted and combined.
Take it off the heat and add the lemon syrup, lemon zest and a drop or two of yellow food colouring (if using). Mix to combine.
Pour the mixture into the lined baking tin. Top with sprinkles.
Leave for 30 minutes to cool at room temperature, then cover with plastic wrap and refrigerate overnight.
Cut into squares and serve.