In a large mixing bowl whisk together baking mix, sweetener and salt. Set aside.
Preheat oven to 325 degrees. Line a 16" x 11" sheet pan with parchment paper and set aside.
In a medium saucepan melt butter. Add cocoa and stir until fully incorporated. Add boiling water and bring mixture to a boil. Immediately remove from heat. Pour over dry ingredients and mix until incorporated.
In a small mixing bowl whisk together buttermilk, eggs, vanilla and baking soda. Add to cake mixture and pour into prepared sheet pan. Bake for 25 minutes or until a toothpick inserted in the middle is clean.
For frosting place butter in a sauce pan and over medium heat melt. Add cocoa and mix well. Remove from heat and add heavy cream, vanilla and sweetener. Mix well. Add pecans and spread over cooked cake.
Slice and serve warm for extra gooey pieces or let set for about 20 minutes before serving.
For the cake I used granulated Swerve. For the frosting I used the confectioners style.