Preheat oven to 350 degrees.

Prepare an 8 or 9″ square pan with a parchment sling, then lightly grease the sides of the pan.

Make the filling: In a small measuring cup or bowl, combine the sugar, cinnamon and cocoa. Set aside.

Make the streusel:

Measure flour by fluffing it up in the bag and then spoon into a cup, 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  your fingers, work in the butter until crumbs form and no dry spots remain. You should have little crumbs and big crumbs. I actually added 2 more T of butter because my mixture was dry.

Make the cake:

In the bowl of a stand mixer, combine the flour, sugar, baking powder, salt and baking soda. Add the butter and mix at low speed, until the mixture looks sandy, about 1 to 2 minutes. (This is known as reverse creaming.)

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

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

Transfer half the batter into the prepared pan-about 1.5 cups, spreading it all the way to the edges using an offset spatula.

Sprinkle all the filling evenly over the batter taking care that it reaches the corners.

Scrape the rest of the batter over the top of the filling using a rubber spatula. Smooth the batter over the filling.

Sprinkle the streusel evenly over the batter and gently press them to adhere to the batter.

Bake the cake for 45 to 50 minutes until it’s set in the middle and light brown on the edges. Check with a toothpick inserted into the center. Only a few moist crumbs should be clinging to it.

Remove the cake from the oven and allow it to cool in the pan for 20 minutes.

Prepare the glaze now if using. Stir together the glaze ingredients until smooth, starting with the smallest amount of water. Add additional water as needed until the glaze is the consistency of molasses.

Remove the cake from the pan using the overhanging edges as handles to gently lift it to a wire rack. Let it cool.

After the cake has cooled, drizzle the glaze over the cake or dust with powdered sugar.

Transfer the cake to a serving dish. I cut around the edges to remove the parchment paper.