Preheat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper leaving a 1-inch overhang for easy removal, then lightly butter the parchment or spray with cooking spray.

Toast the pecans by spreading 1 1/2 cups on a baking sheet and baking 6–8 minutes until fragrant and slightly darker; cool and roughly chop. Reserve 1 cup for topping and 1/2 cup to fold into the batter if desired.

In a large bowl, cream together 6 tablespoons softened unsalted butter with 1/2 cup light brown sugar and 1 cup granulated sugar until light and aerated.

Add the eggs one at a time, mixing after each addition, then stir in vanilla extract and the instant espresso powder if using.

Whisk together the dry ingredients — 2 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon fine salt, and 1/2 teaspoon ground cinnamon if using.

Alternate adding the dry mixture and 1/2 cup buttermilk to the creamed mixture, starting and finishing with the dry ingredients; mix only until combined and fold in the reserved 1/2 cup chopped pecans if desired.

Spread the batter evenly in the prepared pan and smooth the surface so the topping will adhere.

For the topping, melt 4 tablespoons butter in a small saucepan and stir in 1/2 cup light brown sugar, 1 tablespoon light corn syrup (optional), and 2 tablespoons heavy cream; whisk until glossy and slightly thickened, then stir in 1 cup toasted chopped pecans.

Pour the warm pecan topping over the batter, spreading and pressing the nuts gently to ensure contact with the surface.

Bake for 30–38 minutes, rotating halfway, until the center is set. Tent with foil if the topping darkens too quickly. Cool in the pan 20–30 minutes before removing to finish cooling.

Optionally dust the top with powdered sugar and finish with a pinch of kosher salt or a touch of lemon zest for contrast before slicing.

For clean slices, chill briefly and use a sharp, warmed knife between cuts. Serve warm or at room temperature.