Sugar Free Chocolate Cake Too Dense Yes, I know it's almost impossible to get a good sugar-free chocolate cake, but I won't give up trying,as my husband is a diabetic, and I want to make something nice for his birthday.  This recipe is good-tasting, but too dense, with a fudgy layer at the bottom. I've been working on this for quite a while, but I need the help of professionals with this one.  

Sugar-Free Black Magic Cake
Good tasting, but too much moisture, and a fudgy layer at the bottom


½ cup coconut oil (115 g)  2 ¼ cups Splenda (56g) 1 medium banana (½
cup when mashed) (59 g) (try leaving out)  1 teaspoon baking soda  1
large egg plus 3 large egg yolks (117 g total) (save whites for
another use) (try 2 yolks)  1 cup full-fat Bavarian-style buttermilk
(240 g) (try ½ cup lowfat buttermilk)  1 tablespoon pure vanilla
extract  ¾ cups unsweetened natural dark cocoa powder(63 g)  ½
teaspoon baking powder  ½ cup nonfat dry milk powder(g) (try 1 cup) 
1 teaspoon salt  2 teaspoons freshly ground cardamom  1 cup
self-rising flour (125 g), or convert plain flour to self-rising,
sifted twice (try cake flour)  1 cup strong brewed coffee (240 g), or
1 cup water plus 4 teaspoons Nescafe instant coffee  Dark unsweetened
baking chocolate (190 g) (try leaving out)
Preheat oven to 300º F. Grease and flour two double-parchment-lined (bottom only) 8-inch round cake pans with 2-inch sides. Prepare brown paper collars for the pans.  
In a small saucepan set over low heat, melt together baking chocolate, cocoa, coffee and coconut oil, whisking to combine. Set aside. 
Using a fork, mash banana with baking soda on a plate. In the bowl of a mixture, add banana mixture, eggs, Splenda, buttermilk, vanilla and the rest of the ingredients, except flour and chocolate mixture. Whisk thoroughly, using whisk attachment.  
Switch to mixing attachment. Bring chocolate mixture to a boil.  Add flour and boiling chocolate mixture. Mix again on medium speed, scraping down sides and bottom of the bowl, until batter is well mixed, a couple of minutes.   
Pour batter into prepared pan(s). Tap pans gently several times on the counter to remove air bubbles. Bake on middle rack 20 to 25 minutes or just until a toothpick inserted in the center comes out clean. 
When just out of the oven, remove the cake from the pan and place both cake and the parchment it was baked in, on a rack and then straight into the freezer for at least 30 minutes. After this time, wrap with plastic wrap, then foil, and place back into the freezer until ready to frost.  


Notes:

For best flavor and sweetness, banana should have dark spots on the      skin. Try adding ¾ cup cornstarch to lighten the cake 1 ½ recipe for     2 (9”) cake pans.  If using 1 (9”) pan, bake 40-45 minutes.   Wet       ingredients should be at room temperature before mixing. Batter       should be no more than 2” deep Try Bain-marie-Place cake pans into a     bigger pan of water.  If using cake right after cooling, flip the top    layer over so that the domed top becomes the bottom of the cake. Flip    the second cake so that the bottom becomes the top of the cake,       slicing some of the dome off if necessary to make an even cake.  If      making one cake, cut in half horizontally and flip.
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.