Chocolate chip Coffee Cake

From Smitten Kitchen

CAKE

1/2 cup unsalted butter (4 ounces or 115 grams) at room temperature

1 1/2 cups (300 grams) granulated sugar

3 large eggs, separated

1 1/2 teaspoons vanilla extract

2 cups (16 ounces) sour cream

3 cups (390 grams) all-purpose flour

1 teaspoon baking powder

1 1/2 teaspoons baking soda

3/4 teaspoon fine sea or table salt

FILLING AND TOPPING

2 cups or 12 ounces semi- or bittersweet chocolate chips

1/2 cup (100 grams) granulated sugar

1 teaspoon ground cinnamon

Heat oven to 350°F (175°C). Butter a 9-x-13-inch baking pan and line the bottom with parchment paper. 

In a large bowl, cream butter and 1 1/2 cups granulated sugar. Beat in egg yolks and vanilla. Whisk 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 with clean beaters, beat eggs whites until stiff, then fold gently into batter.

In a small dish, combine the cinnamon and remaining 1/2 cup granulated sugar for filling and topping.

Spread half the cake batter in the bottom of prepared pan and spread smooth. 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. 

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

Do ahead: I do *not* cover this when storing it (at room temperature because) I don’t want to lose the crackly top.