• For the Chocolate Cake
• Preheat the oven to 350F. Grease two 8" cake pans with baking spray, line with parchment paper and grease lightly again.
• In a medium saucepan over medium-low heat, melt the chopped chocolate and butter until smooth. Set aside and let cool.
• In a large bowl, combine the dry ingredients: flour, sifted cocoa powder, baking soda, baking powder, sugar, light brown sugar and salt. Whisk until fully incorporated.
• In a separate bowl, whisk the eggs and egg yolk. Add the remaining wet ingredients and beat until smooth.
• Mix the wet ingredients with the dry ingredients, and mix until there are no longer any clumps of flour.
• Fold in the chocolate chips, if using.
• Divide the batter amongst the 2 cake pans. I like to weigh the batter using a scale, to get even sized cakes.
• Bake for 40 minutes, or until a toothpick inserted in the center comes out clean. 
• Allow to cool for 10 minutes and transfer to a cooling rack.
• For the Dulce de Leche Buttercream
• In the bowl of a stand mixer with the paddle attachment, beat the butter and powdered sugar until creamy, about 5-7 minutes.
• Start adding the kefir one tablespoon at a time, until desired consistency (I used 3 tbsp). 
• Add 1/4 of the dulce de leche and beat until incorporated. Add another 1/4 and repeat until you've used up all the dulce de leche.
• Add the vanilla and salt and beat. 
• If the mixture is too liquid, add more powdered sugar. If it's too thick, add another tbsp of kefir until you've reached the desired consistency.

Assemble:

• Place the first chocolate cake layer, flat side up, on a plate or cake stand. Using an offset spatula, spread the top with the dulce de leche buttercream.
• Place the second layer on top, rounded side up, and spread the buttercream evenly on the top. If you want to make swirls like I did, add the remaining buttercream to a piping bag with a star tip. Decorate in a half-moon style and add sprinkles!
• Enjoy!