While I usually bake with Granny Smith apples, they tend to dry out a little in this coffee cake. Honeycrisps or even Golden Delicious are perfect here. You could also use pears instead of apple or walnuts or hazelnuts instead of pecans. I love baking with whole grain flours whenever possible; you want coffee cake to be pretty fluffy, so I opted to use King Arthur Flour’s white whole wheat flour along with a traditional white all-purpose flour. This combination works well and gives the cake a little more heft without verging on dry or overly dense.
Preheat the oven to 350 F. Grease a 9 x 13-inch baking pan. With a stand-mixer or hand-beaters, beat the butter until creamy, roughly 1 minute. Gradually beat in the two sugars until fluffy, 2-3 minutes. In a medium-sized bowl, combine the flour, cinnamon, nutmeg, baking powder, baking soda and salt. Whisk together with a fork or small whisk. Set aside.
Add the eggs to the butter mixture, one at a time, beating after each addition just to incorporate. Add the four mixture, alternating with the buttermilk, beginning and ending with the flour. Fold in the apples just until combined and scrape the mixture into the prepared pan. This is the point in which you really don’t want to overmix the batter: just fold and be done with it.
For the topping: Mix all of the dry ingredients together, pour in the butter and work the butter into the dry ingredients with your hands until crumbly. It should be clumpy, like large pebbles. If it’s too wet, add another sprinkling of flour.
Scatter the topping over the coffee cake batter in an even layer. Bake for 50 minutes, or until a toothpick is inserted into the middle and comes out clean or with just a few crumbs attached. Cool completely before serving.