My dear MIL is also diabetic, and I make this cake for her often.

Diabetic Chocolate Cake
Stir together:
1 1/2 c. flour
1/2 c. sugar
3 Tbsp. cocoa
1 1/2 tsp. baking soda
12 packets Sweet & Low
Add:
1 c. water
1 egg
1 Tblsp. white vinegar
2 tsp. extract of choice (vanilla, rum, butter, etc.)

Stir until smooth. Spray a 9-inch cake pan with non-stick cooking spray. Pour batter into pan. Bake at 350 degrees for 18 minutes, or until tester inserted in center comes out clean. Cake will spring back lightly when touched. Cool in pan on rack for 10 minutes. Turn out on rack to cool completely. 1/6 of cake = 1 serving.

I just dust with powdered sugar for her. I'm sure you could use any sugar free topping of choice to jazz it up a bit.