Updated Recipe (older one below)
This frosting is reminiscent of Betty Crocker® Frosting, but I think it is nicer.

3 tbsp butter, melted (45 mL)
2 tbsp sugar-free pancake syrup (30 mL)
Liquid sweetener to equal 1/2 cup (125 mL)
  Sugar, OR to taste
1/4  cup sifted, powdered erythritol (60 mL)
1/4  cup whole milk powder (60 mL)
1/2  tsp cornstarch (2 mL)

In blender, combine butter, sugar-free pancake syrup, liquid sweetener to taste, powdered erythritol, whole milk powder and cornstarch.  Blend until smooth.  If the frosting is too thick, add water 1 tsp (5 mL) at a time and blend.  If the frosting is too soft, let it sit for half an hour and it should thicken up.

Helpful Hint:  Instead of sifting the erythritol, you could place it in a coffee bean grinder or blender to make sure there are no lumps. 

Yield:  1 batch
1 batch (total amount)
469.2 calories
8.8 g protein
43.1 g fat
13.5 g net carbs

Betty Crocker-like Frosting:
(The low-carb pancake syrup, such as Estee, Maple-Flavored Pancake Syrup, has thickeners in it and it makes this frosting a thicker mixture)
1/2 cup Splenda Granular (125 mL) or use zero carb liquid sweetener
4 tbsp powdered erythritol (50 mL)
4 tbsp whole milk powder* (50 mL)
1 tsp cornstarch (optional) (5 mL)
2 tbsp low-carb syrup (25 mL)
3 tbsp butter, melted (45 mL)

In blender, combine Splenda Granular, powdered erythritol, whole milk powder and cornstarch (if using). Add sugar free syrup and butter; blend until well combined. If the frosting is not thick enough, then use a tiny amount of Xanthan gum.

The dry mixture up to cornstarch in the listing above can be stored in the refrigerator until required. It will rival the real thing any day! No one will guess and that's the best part. The cornstarch is totally optional and not necessary - I was just trying to mimic the taste of the real thing but you'll hardly miss it. Thing is though it only saves 0.2 g carbs per tbsp (15 mL) by leaving it out.

Yield: 3/4 cup (175 mL) delicious frosting. Double the recipe for 1½ cups (375 mL) frosting. The frosting is slightly maple-flavored.
3/4 cup (175 mL) = 12 tbsp, 1 tbsp:
43.9 calories; 0.7 g protein; 3.6 g fat; 2.3 g carbs

Helpful Hints: This recipe can be multiplied several times. One could keep the dry ingredients (mixed) in a container in the refrigerator or freezer for a long time. Whole milk powder can be found in Publix or the Hispanic section of a Super Walmart. *Instead of whole milk powder, use skim milk powder but first blend finely in a blender before using to prevent lumps in the frosting.

The above amount will frost two mini loaves, one regular loaf, bundt cake or one single-layer 8 or 9 inch cake.

Nutritional analysis For two mini loaves (10 slices per loaf)
1.4 g carbs

16/25 servings (8-inch square cake)
1.7/1.1 g carbs

18 servings (1 large loaf or tube cake)
1.5 g carbs

1 Batch:
24.3 g carbs