Begin preheating the oven to 350° F (176° C) and lightly spray a 9” square baking pan with cooking or baking spray.
Topping:
Melt the butter in a small dish. Combine the flour, brown sugar, cinnamon, and salt in a small bowl then pour the melted butter over. Mix everything together with a fork until well combined. Using your hand, flatten and compact the topping mix in the bowl until it is packed well. Place the topping mix in the refrigerator to chill.
Cake:
Combine the flour, baking powder, and salt in a bowl, fluffing lightly with a fork. Stir together the sour cream (or yogurt) and milk in a measuring cup.
In the bowl of a mixer, cream the butter and sugar together for about 2 minutes on medium to medium high speed until nice and fluffy.
Set the mixer to stir or low and add one egg, incorporate well, then add the second egg and stir well.
Pour the sour cream/milk into the mixer bowl and increase the speed slightly to combine.
Turn the mixer back to low or stir and steadily add the flour mix in. Once it’s all in, stir until barely mixed to avoid over mixing. Scrape the bowl, pour in the chocolate chips, and gently fold to distribute by hand with a spatula. The batter will be quite thick.
Scoop the batter into the greased pan and spread it level.
Remove the topping mix from the refrigerator and with a dinner knife, break it into various sized chunks. Empty the bowl on top of the coffee cake batter and spread the chunks out evenly, gently pressing them very slightly into the batter.
Place the pan in the oven and bake for 35-40 minutes. A toothpick tester will have a few moist crumbs on it. Cool the coffee cake on a rack completely before serving.