Microwave butter in a 2-cup glass measuring cup at HIGH for one minute, or until butter is melted. Stir in brown sugar, water, and cornstarch. Microwave again at HIGH for one minute, or until mixture is boiling. Stir in chopped pecans. Spread mixture evenly onto the bottom of pie crust. Place crust in freezer while you prepare chocolate mousse.
Place gelatin powder in a small bowl, sprinkle cold water over it and let it sit for 1 minute. Pour in boiling water and whisk together until gelatin is completely dissolved. Set aside.
In a medium-sized bowl, add sugar and cocoa powder. Whisk to combine. Add cold heavy cream and vanilla, and beat using an electric hand mixer on medium speed, until mixture stiffens up. Add gelatin mixture and beat until well-incorporated.