In a bowl, whip the heavy cream to soft peaks--not too stiff.
In a large bowl, using a hand-held mixer, whisk the eggs until thick, about 2 minutes
Combine the sugar and water and bring to a boil for 1 minute. Pour the sugar mixture over the yolks, and beat mixture again.
In a bowl set over a saucepan of simmering water (the water should not touch to bottom of the bowl), melt the chocolate and butter, stirring occasionally. Remove the bowl from the heat and let cool until just warm.
Fold in the egg mixture and whipped cream into the chocolate mixture until just combined. Refrigerate the chocolate mousse until set, about 2 hours.
To serve: Set an ice cream scoop in a bowl of hot tap water for a few minutes.
Lay a stencil cut from a piece of parchment paper (heart, snowflake, Easter egg, or Santa's boot), on a plate and sprinkle cocoa powder over the top. Remove the stencil and repeat on the remaining plates.
Using the hot ice cream scoop, smoothly scoop the chocolate mousse and center a scoop on each plate. Decorate each plate with orange segments and mint.