To prevent fingerprints or smudges, wear thin cotton gloves* while handling the eggs.
Makes 3 large and 4 small eggs.
Make chocolate shells:
Using rubber spatula, stir chocolate in large metal bowl set over saucepan of simmering water until smooth and candy thermometer registers 115°F. (do not allow bottom of bowl to touch water). Remove from over water.
Place 1 tray of large egg molds on work surface; if necessary, place folded dish towel under tray to level. Spoon chocolate into egg molds, filling completely (do not fill embossed mold). Quickly invert tray over bowl of chocolate, allowing excess chocolate to drop into bowl. Shake tray gently to produce uniformly thick chocolate shells. Run fingertip around edges of shells to remove excess chocolate. Chill tray.
Set bowl of chocolate back over simmering water and re-warm to 115°F., stirring frequently. Repeat coating and chilling process with remaining trays ofegg molds, rewarming chocolate over simmering water to 115°F. after coating molds in each tray. Reserve remaining chocolate at room temperature in bowl for use in filling and assembly.
Make chocolate-toffee filling: Stir cream and butter in heavy medium saucepan over medium heat until butter melts and mixture comes to boil. Remove from heat. Add 1 2/3 cups reserved melted chocolate; whisk until smooth. Whisk in sour cream and vanilla; fold in toffee bits. Let stand at room temperature until cool and slightly thick but still pourable, stirring occasionally, about 1 hour.
Spoon enough filling into each chilled chocolate shell to come within 1/8 inch of top edge, about 6 tablespoons for large chocolate shells and 3 tablespoons for small chocolate shells. Refrigerate until filling is firm, about 2 hours.
Rewarm remaining chocolate in bowl over simmering water until thermometer registers 115°F., stirring frequently. Remove from over water. Quickly spoon enough melted chocolate over filling in each shell just to fill. Using offset spatula, spread chocolate over filling to seal to edges, scrape off excess chocolate and level top. Refrigerate all trays until top layer of chocolate is very firm, at least 2 hours. Reserve remaining chocolate at room temperature in bowl.
Place large sheet of foil on work surface. Turn 1 tray over just above foil. Twist and bend tray to release egg halves onto foil. Unmold remaining egg halves.
Rewarm chocolate in bowl over simmering water until thermometer registers 115°F., stirring frequently. Remove from over water. Turn bottom egg halves (those with small flat base) so that large flat side faces up. Spread small amount of melted chocolate over large flat side of 1 egg bottom, leaving 1/4-inch border at edge. Immediately place corresponding egg top on chocolate and press gently to adhere. Wipe away any excess chocolate from seam. Place whole egg on foil-lined baking sheet and refrigerate. Repeat with remaining egg halves and melted chocolate. (Assembled eggs can be prepared 2 weeks ahead. Cover; keep refrigerated.)
Prepare icing according to package directions; gradually mix in enough brown gel to reach desired shade. Spoon some icing into pastry bag fitted with star tip.
Place 1 chocolate egg on revolving cake stand or on small round of foil on work surface. Holding tip of pastry bag close to egg, pipe 1 continuous line of icing decoratively onto seam, turning cake stand or foil as necessary. (Do not pipe unconnected designs such as stars onto seam; they break off easily.) To attach purchased icing decorations, use a toothpick to dab melted chocolate onto bottom of decoration, then affix to egg.
Return decorated eggs to foil-lined baking sheet. Chill until icing sets, about 2 hours. (Can be made 1 week ahead. Keep chilled in large plastic containers.)