• 1/2 cup (1 stick) unsalted butter at room temperature
  • 1 1/2 cups granulated sugar
  • 3 large eggs, separated
  • 1 1/2 teaspoons vanilla extract
  • 3 cups all-purpose flour
  • 1 1/2 teaspoons baking soda
  • 1 teaspoon baking powder
  • 3/4 teaspoon table salt
  • 2 cups sour cream
  • Filling and Topping
  • 2 cups or 12 ounces semi- or bittersweet chocolate chips or coarsely chopped chocolate bars
  • 1/2 cup granulated sugar
  • 1 teaspoon cinnamon
Preheat oven to 350degF. Butter a 9-x-13-inch baking pan. If not nonstick coated, cover the bottom with a rectangle of parchment paper. Set pan aside.

In a large bowl, cream butter and 1 1/2 cups sugar. Beat in egg yolks and vanilla. Sift flour, baking soda, baking powder and salt together into a separate bowl. Alternately mix in sour cream and then dry ingredients into butter mixture until both are used up and the batter is smooth and very thick. In a medium bowl, beat eggs whites until stiff, then fold into batter.

In a small dish, whisk together sugar and cinnamon for filling and topping.

Spread half the cake batter in the bottom of prepared pan. Sprinkle with half of cinnamon-sugar mixture and 1 cup of chocolate chips. Dollop remaining cake batter over filling in spoonfuls. Use a rubber or offset spatula to gently spread it over the filling and smooth the top. Sprinkle batter with remaining cinnamon-sugar and remaining chocolate chips. With the palm of your hand, ever-so-gently press the chocolate chips a bit into the batter. No need to submerge them, you just want to make sure they adhere bit.

Bake for 40 to 50 minutes, rotating halfway through, until a tester inserted into the center of the cake comes out clean.