Sugar Free Chocolate Cake Too Dense
There's a few things that could be happening here, you could have too much moisture in the batter, not enough leavening agent (unlikely in this case) but my money is the oven temperature - it seems low. I'd usually bake a cake at 350F. 

When baking a cake you first get a rise from the action of the leavening agents and the expansion of air and water vapor due to heat, next a crystalline structure forms which holds that shape. If your oven is too low that crystalline structure won't form in time to hold the rise, the leavening agents will run out and the steam and air will escape, and your cake will collapse leaving it dense. 

In addition to the oven being low the baking time might be too short, if the baking time is too short the batter won't have time to crystallize and it will collapse when taken out of the oven, again making it dense. 

I'd try upping the temperature a bit and see how it goes. Also, the toothpick check isn't the most reliable, use a touch bounce test and a digital instant read thermometer. Most cakes are done when they read between 200-210F in the middle of the cake.