Melt 6 tablespoons of the butter. Combine cookie crumbs and butter in a small bowl and mix until thoroughly moistened.
Transfer to an 8-inch tart pan. Using the back of a cup or measuring cup, press mixture into the pan with a removable bottom until evenly covering the bottom and sides of the pan. Set in freezer until set, at least 20 minutes.
Meanwhile, place chocolate in a medium heatproof bowl and set aside. Place cream in a small saucepan and bring to a simmer over medium-low heat.
Remove from heat and whisk in malted milk powder until smooth and pour over chocolate. Let sit until chocolate is starting to melt, about 1 minute. Gently stir until all chocolate is melted and smooth. Stir in remaining 2 tablespoons of butter, small piece by piece, until evenly incorporated.
Turn mixture into the prepared tart crust and spread until even. Cover loosely and refrigerate until filling is set, at least 1 1/2 hours.