Preheat oven to 350. Coat three 6 cup popover or muffin pans with nonstick spray.
For the topping mix chopped espresso beans and chocolate chips. Set aside. For the filling, process 1/4 cup sugar, 1/4 cup espresso beans, cocoa, and chopped chocolate in a food processor til sandy.
For the cake, whisk together flour, baking powder, salt, and baking soda in a bowl. Beat butter and vanilla with mixer in bowl til smooth.
With mixer running, gradually beat in 1 2/3 cup sugar til light and fluffy and all sugar dissolves, about 5 minutes
Stir together coffee and espresso powder, then stir into buttermilk. Alternately mix flour mixture and buttermilk mixture into creamed mixture starting and ending with flour, just to mix
Scoop batter with a # 30 scoop (2 Tbsp) into each prepared cup. top batter with 1 heaping tsp. filling and a second scoop of batter. Mix remaining filling with topping and sprinkle on cakes. Bake cake til a toothpick inserted into centers comes out clean,17 to 20 minutes. Let cakes cool in pans, 30 minutes, then loosen edges and remove.