Preheat oven to 350°F (180°C).
For topping: In a 10-inch cast-iron skillet, bring pecans, butter, brown sugar, corn syrup, lemon juice, and salt to a boil over medium heat, stirring frequently until sugar dissolves; cook, stirring occasionally, until slightly thickened and bubbly, 2 to 3 minutes. Remove from heat.
For cake: In the bowl of a stand mixer fitted with the paddle attachment, beat butter and brown sugar at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla. With mixer on low speed, add flour to butter mixture in three additions alternately with buttermilk, beginning and ending with flour mixture, beating just until combined after each addition.
Spray sides of skillet with baking spray with flour. Spoon batter onto topping, gently smoothing top with an offset spatula. Place skillet on a baking sheet.
Bake until a wooden pick inserted in center comes out clean, 35 to 40 minutes. Let cool for 5 minutes. Place a serving plate on cake; carefully invert cake on plate. Let cool for 15 minutes before serving.