1- Start by toasting the shelled pistachios in a dry skillet over medium heat, stirring frequently, until fragrant and lightly browned. This step brings out the natural oils and enhances the pistachio flavor.

2- Next, make the pistachio mousse base by blending the toasted pistachios, powdered sugar, and a splash of vanilla extract in a food processor until it forms a smooth, creamy paste.

3- In a separate bowl, whip the heavy cream until it holds stiff peaks. Gently fold the whipped cream into the pistachio mixture until fully incorporated.

4- Pour the pistachio mousse into individual ramekins or a single serving dish and refrigerate for at least 2 hours, or until set.

5- While the mousse is chilling, prepare the chocolate ganache topping. In a double boiler or heatproof bowl set over simmering water, melt the milk and dark chocolate together, stirring frequently, until smooth and glossy.

6- Remove the set pistachio mousse from the fridge and spoon or pipe the warm chocolate ganache over the top. Refrigerate again for 30 minutes to allow the ganache to firm up.