Combine the gelatin with 2 tbsps of room temperature water.
In a small saucepan combine the water, corn syrup, cream, and cocoa powder. Let the mixture come to a simmer, stirring consistently.
Place the gelatin in the hot glaze. Whisking until the gelatin is completely melted into the glaze. Pour the glaze through a fine mesh strainer to remove any chunks of cocoa powder from the glaze.
Let the glaze cool to about 90° degrees, stirring occasionally so that a skin doesn't form.
Remove the mousse from the freeze and from the mould. Place the mousses on a cooling rack with a jelly roll pan underneath.
Pour the glaze over the top of the mousses. Covering the top completely.
Let the glaze set on the mousses for 5 minutes. Remove the mousses from the cooling rack with a offset spatula. Place the mousses on the cake bases.
Let the petit gateau set in the refrigerator for at least 30 minutes.
The petit gateau's will keep in the refrigerator in an airtight container for up to 2 days.