With an electric mixer on high, whip the heavy cream in a bowl just until frothy and add in the 1/3 cup sugar substitute and vanilla and continue to whip just a little more to blend; do not whip all the way to soft peaks yet. Put half of the partially whipped mixture into another bowl and finish whipping it on high speed until stiff peaks form. Be careful not to overwhip, or the cream will break. Set aside the vanilla cream.
To the first bowl with the other half of the partially whipped cream mixture, add in the cocoa powder and 2 tablespoons more sugar substitute and finish whipping on high until stiff peaks form, again being careful not to over-whip. This is the chocolate cream.
In each of 4 parfait glasses, layer a couple spoonfuls of chocolate cream, followed by some fresh strawberry slices, and top it with a couple spoonfuls of fresh vanilla cream. Chill completely before serving. Garnish with a strawberry fan and sprig of fresh mint.