Adjust an oven rack to the middle position. Preheat the oven to 350F. Butter and flour two 8 by 2-inch square cake pans (see note above for other pan options) and line the bottoms with parchment paper.

In a medium bowl or liquid measuring cup, whisk the eggs, egg yolks, vanilla, sour cream, and milk.

In the bowl of a stand mixer fitted with a paddle, mix the flour, sugar, baking powder, baking soda, and salt on low until combined. With the mixer running on low, add the butter one piece at a time, beating until the mixture resembles coarse sand. With the mixer still running on low, slowly add half the wet ingredients. Increase the speed to medium and beat until incorporated, about 30 seconds. With the mixer running on low, add the rest of the wet ingredients, mixing until just combined. Increase the speed to medium and beat for 20 seconds (the batter may still look a little bumpy). Scrape down the sides and bottom of the bowl, add the chocolate chips, and use a spatula to mix the batter a few more times until the chips are incorporated.

Pour the batter into the prepared pans and smooth the top. Tap the pan gently on the counter 2 or 3 times to help get rid of any bubbles. Bake 15-18 minutes, rotating the pan halfway through, until the cake is golden brown and pulls slightly away from the sides and a wooden skewer or toothpick inserted in the centers comes out clean.

Transfer the cakes to a wire rack and let cool for 30 minutes. Turn the cakes out onto a half sheet pan lined with parchment paper, remove the parchment paper from the bottom of the cakes, and let the cakes cool completely upside-down (this will help deflate any doming). Once cool, the cakes can be wrapped in plastic and refrigerated overnight or frosted.

Put about 1 inch of water in a medium saucepan and bring it to a gentle boil. Melt the chocolate in a heatproof bowl set over the pan of boiling water, being careful not to let the water touch the bottom of the bowl. Stir constantly until just melted and set aside to cool slightly.

In the bowl of stand mixer fitted with a paddle, beat the butter on medium until light yellow and fluffy, about 3 minutes. Add the vanilla, corn syrup, and salt and beat on medium until combined. Turn the mixer to low and gradually add the confectioners’ sugar. Beat at medium, stopping to scrape down the sides of the bowl as necessary, until smooth and creamy, 2-3 minutes. Add the chocolate and mix on low speed until no streaks remain.

Use half the buttercream on each chocolate chip snack cake, and frost as desired. Decorate with sprinkles if also desired.