Prepare cake: Heat oven to 350 degrees. Butter two 9-inch cake pans; line the bottoms with wax paper and then butter the paper.
Combine the flour, cocoa, salt, baking powder and baking soda in a small bowl; set aside. Beat the butter in a mixer bowl until creamy, then add the sugar and vanilla and mix. Add the eggs one at a time, then beat two minutes after they're added.
Add the dry ingredients alternately with the sour cream, beating just until blended. Spoon the batter into the pans evenly, spreading the tops smooth (batter will be thick).
Bake about 16-18 minutes or until a toothpick comes out clean from the center. DO NOT OVER BAKE. Cool on racks for 5 minutes, then invert and remove wax paper. Invert again on racks and cool completely.
Make Peanut Brittle: Place a silicone liner on a cookie sheet (or a greased piece of foil). Combine sugar, water and cream of tartar in a small saucepan. Bring to a simmer over medium heat. Cover and simmer 3 minutes.
Remove cover and cook until syrup is pale amber in color, 8 to 12 minutes. Stir in peanuts and cook until syrup is a deep amber, being careful not to burn (only a few minutes). Pour right away onto prepared sheet, spreading as thin as possible. When completely cool, break into small pieces.
Make Peanut Butter Frosting:Combine all frosting/filling ingredients in a bowl of a mixer and beat until well blended.
Make Ganache:Place chocolate, butter, and honey in a microwave-safe bowl and microwave for 1 minute. Stir, and microwave for 10 to 20 seconds more, if needed for all the chocolate to melt (alternately melt in a double boiler, stirring until melted). Cool by placing in the refrigerator while assembling the cake.
Place one layer on a serving plate and frost with 1 cup of peanut butter frosting. Top with second layer and spread top and sides with remaining frosting. Refrigerate about 10 minutes until frosting is set.
Remove ganache from fridge and whisk until smooth. Tuck strips of wax paper under the cake and pour the ganache onto the center top of the cake, letting it flow over the sides. Spread with a metal spatula to cover top and sides, using any that flows onto paper strips, if needed.
Remove paper strips and garnish the top with the peanut brittle pieces.
To Make Ahead: prepare cake, cool and freeze for up to a month. Prepare brittle, cool and keep in at room temperature for a day. Frostings are best prepared the day of serving.