Step 1

Heat the oven to 350°F. Lightly brush the bottom of a 13x9-inch baking pan with room temperature butter. Line the bottom and 2 long sides of the pan with a sheet of parchment paper, making sure the paper extends over the sides by a few inches to form a sling. Lightly brush the bottom and sides of the parchment with more butter. (If you want to serve the cake directly from the pan, skip the parchment and just lightly butter the pan.)

Step 2

Whisk 3 cups all-purpose flour, 2 teaspoons baking powder, and 1/2 teaspoon baking soda together in a medium bowl. Whisk 1 cup room temperature whole milk and 1/2 cup room temperature sour cream together in a bowl or liquid measuring cup.

Step 3

Beat 2 sticks room temperature butter, 1 3/4 cups packed light brown sugar, and 1/2 teaspoon kosher salt with the paddle attachment in a stand mixer on medium-high speed, scraping down the sides of the bowl a few times, until lightened and creamy, 2 to 3 minutes total. Beat in 3 room temperature large eggs one at a time, beating until each one is fully incorporated before adding the next. Beat in 1 tablespoon vanilla extract and 1/2 teaspoon maple extract if desired.

Step 4

Reduce the speed to low. Beat in about one-third of the flour mixture until just combined. Add half the milk mixture and beat until combined (it’s ok if it looks slightly curdled). Repeat beating in the flour in two more increments, alternating with the remaining milk, scraping down the sides of the bowl as needed. Use a flexible spatula to finish mixing, making sure to scrape any dry flour bits from the bottom and sides of the bowl.

Step 5

Scrape into the baking pan and spread into an even layer. Smooth out the top.

Step 6

Bake until the cake is lightly browned on top and the center springs back to a gentle touch, 35 to 50 minutes. Let cool in the pan while you make the frosting.

Step 7

Place 2 cups packed light brown sugar, 1 1/4 cups heavy cream, 3/4 cup maple syrup, and 1/4 teaspoon kosher salt in a large high-sided saucepan (at least 3 quarts). Cook over medium-low heat, stirring frequently, until the sugar is dissolved and the edges begin to bubble, 4 to 5 minutes.

Step 8

Attach a candy or deep-fry thermometer to the side of the pan and increase the heat to medium. Bring to a boil without stirring. Continue to boil until the mixture registers between 234°F and 238°F, 10 to 15 minutes. The sugar will bubble up vigorously. If it gets too close to the top of the pot, reduce the heat a bit but make sure it’s still boiling.

Step 9

Remove the pot from the heat. Add 1/2 teaspoon vanilla extract, then add 3 tablespoons cut-up unsalted butter. Do not stir or disturb the pot. Let cool until warm but not hot (120°F to 130°F), 45 to 55 minutes.

Step 10

Remove the thermometer. Stir with a stiff spatula or wooden spoon until fully blended and shiny. Transfer to a stand mixer and beat with the paddle attachment on medium speed until the color lightens to a light beige (like milky tea) and thickens enough that it can still be poured but a ribbon of the frosting will sit on top of the mixture without melting back into the pot and losing its shape, 30 to 35 minutes. (Alternatively, beat directly in the saucepan with an electric hand mixer, which will take 10 to 15 minutes.)

Step 11

Transfer onto the middle of the cake. Gently spread with a small offset spatula, stopping just before the edges as the frosting will spread just a little on its own. Sprinkle with 1/3 cup chopped, toasted pecans. Let sit until the frosting sets and is firm, 1 to 2 hours. Grasping the excess parchment, lift the cake slab out of the pan onto a cutting board before cutting.