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1
Adjust oven rack to lower-middle position and preheat to 350u00b0F (180u00b0C). Grease a half-sheet pan, dust with cocoa, and tap out the excess.
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2
Combine brown sugar, butter, baking soda, salt, cinnamon, and vanilla extract in the bowl of a stand mixer fitted with a paddle attachment. Mix on low to moisten, then increase to medium and cream until fluffy and light, pausing to scrape the bowl and beater halfway through, about 5 minutes. With the mixer running, add the eggs one at a time, letting each fully incorporate before adding the next and scraping the bowl as needed.
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3
Meanwhile, sift together the flour, cocoa, and malted milk powder (if using). Reduce mixer speed to low and sprinkle in a third of the dry mix, followed by a third of the buttermilk. Alternate between the two, allowing each addition to roughly incorporate before adding the next. Once batter is smooth, fold with a flexible spatula to ensure it's well mixed from the bottom up. Scrape batter into the prepared pan, then spread into an even layer.
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4
Bake until the cake is puffed and firm to the touch, though your finger will leave an indentation in the puffy crust, about 25 minutes.
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5
While the cake is baking, combine the milk, butter, sugar, cocoa powder, and salt in a 2-quart stainless steel saucier. Warm over medium-low heat until the butter is melted, then increase heat to medium and bring to a boil, whisking from time to time, until the mixture is homogeneous. Continue cooking until the syrup reaches 220u00b0F (104u00b0C), then shut off the heat and stir in the vanilla. Cover and set aside to keep warm until the cake comes out of the oven.
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6
As soon as the cake is done, immediately pour the warm glaze on top, working slowly to prevent overflow and using an offset spatula or the back of a spoon to smooth it into an even layer. Cover all over with toasted pecans, as few or as many as you like. Enjoy warm, or let it stand at room temperature until the glaze has fully set. Wrapped in plastic, the glazed cake will keep up to 1 week at room temperature.
