In a large glass bowl, place the chopped white chocolate and set aside.
Add the egg yolks and sugar to a small bowl and whisk until pale in color.
In a saucepan, over low heat, bring 1/4 cup of the cream to a simmer, and slowly add the cream into the yolk and sugar mixture to temper. Pour the creamy mixture back into pan and stir with a wooden spoon until it coats the back of it.
Pour hot mix into a fine mesh sieve placed directly over bowl with the chopped chocolate. Stir until completely smooth.
In another bowl, whip remaining 1 cup of the cream to almost stiff peaks. Fold half the whipped cream into the white chocolate mix to lighten and then fold in the remaining whipped cream.
Spoon the white chocolate mousse into 4 serving cups and refrigerate until set, approximately 1 hour.
Garnish each serving with 3 blackberries and a sprig of mint, if using.