Preheat oven to 325ºF and grease a mini sheet pan (9.5 x 6.5 inches) very well. You can also bake this cake in an 8x8 inch metal pan.
In a medium bowl, whisk together the almond flour, sweetener, protein powder, baking powder, and salt. Break up any clumps with the back of a fork.
In a small saucepan over medium heat, combine the butter, water, and cocoa powder, stirring until melted. Bring to a boil and then remove from heat. Add to the bowl.
Add the egg, vanilla extract, and cream, and stir until well combined. Spread the batter in the prepared baking pan.
Bake about 15 minutes until the top of the cake is just firm to the touch.
In the same saucepan, combine the butter, cocoa powder, cream, and water. Bring to a simmer, stirring until smooth. Stir in the vanilla extract.
Add the powdered sweetener and whisk vigorously to dissolve any clumps. Whisk in the glucomannan and let cool a few minutes to thicken.
Pour over the warm cake and sprinkle with the pecans. Let cool until the glaze is set, about 1 hour.
