Step 1

Preheat the oven to 350°F. Prepare an 8" square pan with a parchment sling or reusable silicone liner, then lightly grease the sides of the pan. Alternatively, line a 9" cake pan with a round piece of parchment or round reusable liner and lightly grease the sides.

Step 4

, To make the filling : In a small bowl, combine the sugar, cinnamon, and cocoa. Set aside.

Step 5

, To make the crumbs : Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess. In a medium bowl, whisk together the flour, sugars, cinnamon, and salt. Add the butter and vanilla; toss to combine. Using a bowl scraper, fork, or your fingers, work in the butter until crumbs form and no dry spots remain. The crumbs should be irregular, ranging in size from a pea to walnut. Set aside.

Step 9

, To make the cake: In a large bowl or bowl of a stand mixer, combine the flour, sugar, baking powder, salt, and baking soda. Add the butter and mix at low speed using an electric hand mixer or the flat beater attachment of a stand mixer until the mixture looks sandy, about 1 to 2 minutes. (This mixing method is called reverse creaming and is also known as the paste method.)

Step 13

, Beat in the vanilla and eggs, then mix until no dry spots of flour remain. Stop the mixer and scrape the bowl and beater attachment as needed; the mixture will be thick.

Step 16

, If using buttermilk, slowly pour it into the mixture while the mixer is running on low speed. If using Greek yogurt, add the yogurt all at once before starting to mix. Continue to stir until the batter is smooth. Stop to scrape the bowl and then mix again at low speed for 30 seconds.

Step 20

, To assemble and bake the coffee cake : Transfer half the batter (about 1 1/2 cups, 420g) into the prepared pan, spreading it all the way to the edges. (A small offset spatula is a helpful tool here.)

Step 23

, Sprinkle the filling evenly across the batter.

Step 26

, Transfer the remaining batter atop the filling and gently spread to the edges. (The batter will be pourable if you’ve used buttermilk; otherwise, transfer the thick batter by dolloping portions over the filling then use a spatula to smooth it out into an even layer.) It’s OK if a little bit of the filling gets mixed into the top layer of batter.

Step 30

, Sprinkle the crumbs evenly over the batter and gently press to adhere.

Step 34

, Bake the cake for about 45 to 50 minutes, until it's set in the middle and light brown. When the cake is done, a toothpick or paring knife inserted into the center should come out mostly clean.

Step 35

, Remove the cake from the oven and allow it to cool in the pan for 20 minutes. Prepare the glaze while the coffee cake is cooling, if using.

Step 36

, To make the glaze: In a small bowl, stir together the glaze ingredients until smooth, starting with smaller amount of water. Add additional water as needed until the glaze is the consistency of molasses.

Step 37

, Remove the cake from the pan using the overhanging edges of parchment as handles to gently lift it out. Place the cake (still on the parchment) on a wire rack to cool slightly.

Step 40

, Drizzle the glaze over the cake, or finish by dusting with non-melting sugar or confectioners’ sugar.

Step 44

, Storage information: Store leftover coffee cake, well wrapped, at room temperature for several days; freeze for longer storage.