Prepare Brown Sugar Génoise.
Blanch the peach by placing it in a 1½-quart saucepan filled with boiling water to cover. Turn off heat and let sit for 15 to 30 seconds. Remove the peach and run it under cold water to stop any cooking. Set aside to cool completely. (This can be done several hours before assembling. If so, refrigerate the blanched peach.)
Split the cake in half (review methods, if necessary).
Turn the top layer of the cake upside down, center it on the cardboard round and transfer to the turntable.
Whip the heavy cream with the sugar and vanilla until soft peaks form.
Spread the equivalent of
Center the second layer on top. Slip the cleaned spatula under the cardboard to aid in lifting the cake. Hold in one hand while frosting with the other hand to coat the sides with a thin film as smoothly and evenly as possible. Then return to the turntable to frost over this initial coating and finish the top and sides.