Preheat oven to 350°. Spray a 12-inch cast-iron skillet with baking spray with flour.
Prepare cake mix according to package directions with whole eggs, adding mashed banana, cinnamon, and nutmeg; beat with a mixer at medium speed until thick and smooth, stopping to scrape sides of bowl. Spread batter into prepared pan.
Bake until a wooden pick inserted in center comes out clean, 25 to 28 minutes. Let cool on a wire rack for 5 minutes. Using the handle of a wooden spoon, poke holes all over top of warm cake at 1-inch intervals.
In a large heavy-bottomed saucepan, whisk together sugar, flour, and salt; whisk in milk until smooth. Cook over medium heat, whisking constantly, just until bubbles form around sides of pot. (Do not boil.) Remove from heat.
In a medium bowl, place egg yolks; slowly whisk in half of hot milk mixture. Whisk yolk mixture into remaining milk mixture in pan. Cook over medium heat, whisking constantly, until thickened, 3 to 4 minutes. Remove from heat; whisk in vanilla. Let cool for 10 minutes, whisking occasionally.
Spread warm pudding all over cake. Cover with plastic wrap, and refrigerate until cold, at least 4 hours or overnight.
Just before serving, top with sliced bananas and whipped cream. Garnish with vanilla wafers and pecans, if desired.
Recipe by Paula Deen Magazine at https://pauladeenmagazine.com/skillet-banana-pudding-poke-cake/