Start by combining the gelatine and cold water in a small bowl. Stir with a spoon until the gelatine is fully dissolved to avoid clumps.
Put milk in a medium saucepan and bring to a boil over medium heat. Remove from heat.
Whisk together the egg yolks and sugar in a large bowl until combined. While whisking, slowly pour in a little bit of the hot milk. Whisk until combined.
Pour the eggs and milk back into the saucepan with the remaining milk, whisking as you go. Cook the custard over medium-low heat, gently stirring with a spatula until thick enough to coat the back of the spatula, about 10 minutes.
Remove from heat and while still warm, add the peanut butter and the gelatine. Stir until combined. Cool the peanut butter base to room temperature, about 30 minutes
.In a stand mixer fitted with a whisk attachment, whip the cream on medium speed until it holds soft peaks, about 5 minutes. Slowly pour the peanut butter mixture into the whipped cream a little at a time, gently folding with a rubber spatula until just combined.
Pour the mousse into the ring and smooth the top with an offset spatula. Freeze until frozen solid, 3 hours to overnight.