Firstly make sure that you have fed your sourdough starter the night before enough so that you can make this recipe and still have enough to feed for tomorrows delicious bakes. Also, make sure to set out the butter to come to room temperature before starting.

In a medium saucepan, add sugar, sourdough starter, and whole milk.

Turn on heat to medium-low, and bring to a simmer. Whisk the mixture continuously so that it doesn't scald or burn to the bottom. The mixture will thicken as you cook it. (This may take 3-5 minutes)

Once thickened, remove from heat and add vanilla, stir in to combine.

Pour this mixture into a heat safe bowl, and cover with plastic wrap. press the plastic wrap down so that it touches the top of the mixture on the entire surface. This will prevent a skin from forming.

Let this cool completely to room temperature (1-2 hours.)

Line a 9x13 baking sheet with parchment paper, make sure to oil the pan under and on the parchment paper and up the sides, so that the cake doesn't stick. (I like to cut the parchment paper to the size of the bottom of my pan, and oil up the sides of my pan really well.)

Preheat oven to 375*F.

Separate egg yolks and whites into 2 different mixing bowls. (You will use both parts separately.)

Beat the egg yolks (yellow part) for about 2 minutes on high with a mixer. Slowly add the sugar to the egg yolks.

Then, add the sourdough starter in small amounts at a time to the egg yolk and sugar mixture. Make sure to mix well between each addition. Scraping down sides as needed.

Next add the cocoa powder and baking soda, and gently mix, but don't overmix. Set this mixture to the side while you work on the egg whites.

In the egg white bowl, sprinkle (*optional) the egg whites with cream of tartar. Beat the egg whites on high until they form stiff peaks.

Then, gently fold in 1/3 of the egg whites into the egg yolk batter. Repeat folding in 1/3 of the egg whites into the batter until the egg whites are gone.

Pour the batter into the lined & oiled baking sheet.

Bake the chocolate sourdough cake for 8 minutes. Just until it doesn't jiggle when moved. DO NOT overbake! Be very gentle with the cake each step, because it is delicate and a little sticky.

Let the cake cool in the pan for 10 minutes, then run knife around the edge of the cake to be sure it won't stick. Place a cooling rack on top of the cake, and flip it over to release the cake onto the cooling rack. Remove the parchment paper from the cake, and allow to cool at room temperature completely before assembling.

After the first portion of the frosting has cooled, and your cake has cooled, then you can finish the sourdough frosting.

In a clean bowl in a stand mixer, add softened butter, and the (cooled to room temperature) first component of the frosting. Using a whisk attachment, whisk the mixture until it is light and fluffy. Scrape down sides as needed.

Taste to adjust the frosting. To thicken or sweeten, you can add a little powdered sugar. If it is too thick, you can add a bit of whole milk. I didn't add either to mine, as I thought it as the perfect balance to compliment the cake and chocolate ganache. (You don't want it too sweet.)

Run a sharp knife under warm water, then cut the cake into 24 equal squares. (Make sure to run the knife under the warm water with each cut, rinsing off any cake residue, and keeping the blade warm and wet.)

Separate out each cake square, and either pipe or spoon the frosting onto half (12) of the squares of chocolate cake.

Then, top each one with the other half of the cake square to make a "sandwich." Gently press, so that the frosting will fill to the edges of the cake. You can use a butter knife to make sure the frosting is all the way to the edges and even. (It is ok, if they look a bit sloppy here, because they are going to get covered in chocolate ganache anyways!)

Place these assembled cakes into the freezer for at least 2 hours. (You can leave them overnight if desired.)

Once the snack cakes are frozen, make the chocolate ganache.

Pour chocolate chips into a medium to large bowl (heat safe, and able to fit the size of your snack cakes.)

In a separate heat safe bowl, heat whole milk until it barely begins to bubble, but not boiling. (I do this for 20 seconds at a time in the microwave, but you could do it in a sauce pan on the stove stirring constantly.)

Pour the hot milk over the chocolate chips, and DO NOT STIR! Let this sit for about 1 minute.

Then, after sitting, whisk the chocolate until smooth.

Dip each frozen square in the chocolate ganache and let excess drain.

Set the cakes on parchment paper to cool until the chocolate starts to harden. This chocolate ganache will not harden into a crisp chocolate, but more like a soft coating. (We found better results not dipping the bottom of each snack cake, so that they didn't stick when picking up.) You can place these in the fridge to speed things up if you prefer.

These are fairly large portions for one square, so I recommend cutting them in half with a sharp knife (run through hot water before & between cutting.)

Enjoy the most delicious chocolate desert ever!