Melt the semisweet chocolate in a double boiler, stirring with a rubber spatula to facilitate even melting. Remove the bowl from the heat and add the butter to the chocolate. Stir to melt the butter and mix well. Add the egg yolks, 1 at a time, and whisk to thoroughly combine.

In a separate clean bowl, beat the egg whites to soft peaks. Gradually add the sugar and beat the whites to stiff, glossy peaks (do not overmix or the whites will dry out). Using a large rubber spatula, gently fold the egg whites into the chocolate mixture.

In a separate bowl, whip the cream to stiff peaks. Using a large spatula, gently fold 1/3 of the whipped cream into the chocolate mixture. Repeat with the remaining cream, folding just until the cream is incorporated. Do not overmix.

Line a 9-inch round cake pan with plastic wrap that over hangs by 4 inches on all sides. Sprinkle the pecan pieces on the bottom of the pan. Pour the mousse over the pecans and fold the plastic over the top of the mousse. Wrap the entire pan with plastic wrap and freeze for at least 2 hours and up to 1 day.

Line a small baking sheet with parchment paper. Melt the dark chocolate and place in a small pastry bag fitted with a 1/4-inch tip. (Alternately, place the chocolate in a small resealable plastic food storage bag and cut 1/4-inch from the tip of the bag.) Drizzle the chocolate back and forth over the baking sheet in vertical strokes. Melt the white chocolate and place in another small pastry bag or plastic bag. Rotate the baking sheet 1/4 turn and drizzle the white chocolate over the dark chocolate in perpendicular strokes to form a lattice-like design. Place the sheet pan in the refrigerator to cool. When the chocolate has hardened, break the chocolate lattice into pieces as needed for garnish.

When serving, unwrap the sheba and cut slices with a hot, wet long, thin knife. A slicer works best for this. Place slices in the refrigerator until softened slightly before serving, about 20 minutes. When ready to serve, place the slices onto dessert plates and garnish with a piece of the chocolate lattice. Serve immediately.