01 -
Grab your food processor and throw in the brown sugar, pecans and cinnamon. Give it a few quick pulses until your nuts are roughly chopped but still chunky. Take about 3/4 cup of this mix and put it in a small bowl with the chopped chocolate. That's your filling - set it aside. Now add the cake flour, cocoa powder, cold butter chunks and espresso powder to what's left in the processor. Pulse till it looks crumbly with tiny butter bits scattered throughout. This makes your topping. Put it aside while you work on the batter.
02 -
Put your oven rack in the middle and turn it on to 325°F (165°C). Rub butter all over the bottom and sides of a 9-inch springform pan or 9×9-inch square pan. You can also spray it with cooking spray or use parchment paper if you're worried about sticking. Just set it aside for now.
03 -
Grab a medium bowl and sift the regular flour, baking powder, baking soda, and salt together. This gets rid of lumps and mixes everything evenly. Just set this bowl aside while you handle the wet ingredients.
04 -
In your electric mixer bowl with the paddle attached, beat your softened butter on medium-high for about a minute until it's smooth and lighter in color. Slowly add the white sugar and crank up to high speed, beating until it's fluffy and light, about 2 minutes. This creates little air bubbles that'll help your cake rise. Drop in the eggs one at a time, mixing well after each one and scraping down the sides. Add the vanilla and mix just until it's blended in.
05 -
Turn your mixer down low. Add your dry and wet stuff in stages, starting and ending with dry: first 1/3 of the flour mix until barely combined, then pour in all the buttermilk. Next add another 1/3 of flour mix, followed by all the Greek yogurt. Finish with the last bit of flour, mixing just enough to combine. Don't overmix or your cake will turn out tough.
06 -
Dump half the batter into your buttered pan and smooth it out. Sprinkle your chocolate-pecan filling evenly on top, leaving a little space around the edges so it doesn't leak out while baking. Gently spread the rest of the batter over this filling layer, being careful not to mix them together. Smooth the top, then sprinkle all of your cocoa-pecan topping over everything.
07 -
Stick the pan in your hot oven and let it bake for 50 to 60 minutes. It's done when you can stick a toothpick in the middle and it comes out clean or with just a few moist crumbs. Start checking around 50 minutes since ovens can heat differently. The cake should look set in the middle and the edges should start pulling away from the pan sides a bit.
08 -
Let the cake sit in the pan on a wire rack for 10 minutes. If you used a springform pan, pop off the sides. Let the cake cool completely on the rack, which takes about 2 hours. This cooling time lets the flavors come together properly. Once it's cool, cut it up and dig in. You can keep it covered at room temperature for up to 3 days.