Chocolate Coffee Cake

For Streusel:

all-purpose flour

firmly packed light brown sugar

ground cinnamon

cold unsalted butter, cut into small pieces

mini semisweet chocolate chips

For the cake:

all-purpose flour

cake flour (ok to use regular flour if you don't have)

unsweetened Dutch-process cocoa powder

baking soda

baking powder

salt

unsalted butter

granulated sugar

large eggs

light sour cream

pure vanilla extract