For Pecan Brittle: Spray a baking sheet with cooking spray. In a small heavy saucepan, combine sugar, 1/4 cup water, and cream of tartar. Stir over medium-low heat until sugar dissolves. Increase heat and let mixture boil without stirring until it turns a deep amber color, about 9 minutes. Occasionally brush down sides of pan with a wet pastry brush. Once a deep amber color, add chopped pecans, and swirl to mix. Pour onto prepared baking sheet. Let cool and break brittle into 3 large pieces for the top of the cake and a bunch of smaller pieces for the filling.
For Cake: Preheat oven to 350 degrees. Spray two 9-inch cake pans with cooking spray and line bottoms with parchment paper. Using an electric mixer, combine cake mix, eggs, sour cream, oil, vanilla extract, and almond extract. Beat until well blended, about 3 minutes. Fold in chocolate and pour batter into prepared pans. Bake for 25-30 minutes, or until set on top. Cool cakes in pans for 10 minutes. Invert onto racks to cool completely.
For Frosting: Combine 1/2 cup brown sugar and 3 tablespoons water in a small heavy saucepan. Stir over medium-low heat until sugar dissolves. Increase heat and boil until slightly thickened, about 3 minutes. Remove from heat and cool 5 minutes. Mix in cream.
In a large bowl, beat 3 cups powdered sugar and butter until well blended. Beat in brown sugar mixture. Add in enough additional sugar so that frosting is desired level of sweetness and is a good spreadable consistency.
Place 1 cake layer, flat side up, on a platter or cake stand. Spread with 1 cup of frosting. Top with small pecan brittle pieces. Top with second cake layer, flat side down. Spread remaining frosting on top and sides. Arrange large brittle pieces on top of cake. If not serving right away, store in refrigerator and let stand at room temperature for 1 hour before serving.