Preheat the oven to 350°F. Line a 9×13-inch baking pan with parchment paper, leaving some overhang for easy removal.

In a large bowl, combine the brown sugar, granulated sugar, flour, baking powder, cinnamon, espresso powder if using, and salt.

Add the softened butter and mix until the butter is incorporated and large crumbs begin to form. The mixture should be crumbly but should hold together when squeezed.

Use your hands to squeeze handfuls of the mixture together, then break it into large, chunky pieces. Don't crumble everything too finely, you want those big pieces.

Place the crumb topping in the refrigerator for 10–15 minutes while you prepare the rest of the cake.

In a medium bowl, combine the melted and slightly cooled butter, brown sugar, granulated sugar, cinnamon, flour, and salt.

Stir until smooth and fully combined. The mixture should be thick, glossy, and spreadable. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, or using a large mixing bowl and hand mixer, cream the softened butter and granulated sugar together for 2–3 minutes, until light and fluffy.

Add the eggs and egg yolks and mix until combined.

Then add the vanilla and sour cream and mix until smooth, scraping down the sides of the bowl as needed.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing just until combined.

Add the whole milk and mix briefly, just until incorporated. The batter will be thick.

Spread half of the cake batter evenly into the prepared 9×13-inch pan. Because the batter is thick, use a spatula or offset spatula to gently spread it into an even layer.

Spread the cinnamon brown sugar swirl evenly over the first layer of batter, leaving a small border around the edges.

Add the remaining cake batter and carefully spread it over the cinnamon layer. Take your time so you don't pull too much of the filling into the top layer.

Remove the crumb topping from the refrigerator and scatter the large pieces evenly over the top of the cake. You want a generous layer of chunky crumbs covering the entire surface.

Bake at 350°F for 55-60 minutes, beginning to check for doneness around 50 minutes.

If the crumb topping is becoming too dark before the center of the cake is finished, loosely tent the pan with aluminum foil and continue baking.

The cake is done when the center is set and a toothpick inserted into the cake comes out clean or with a few moist crumbs.

Remove from the oven and allow the coffee cake to cool completely in the pan before slicing.

Dust with powdered sugar before serving, if desired.