Preheat oven to 350°. Spray a 10-inch oven-proof skillet with nonstick baking spray with flour.
For topping: Add pecans, butter, brown sugar, corn syrup, lemon juice, and salt to prepared skillet. Bring to a simmer over medium heat. Cook, stirring occasionally, 2 minutes. Remove from heat.
For cake: In a large bowl, beat butter and brown sugar at medium speed with an electric mixer until fluffy, 3 to 4 minutes, stopping occasionally to scrape sides of bowl.
Add eggs, one at a time, beating well after each addition. With mixer on low speed, add flour to butter mixture in thirds, alternating with buttermilk, beginning and ending with flour mixture. Beat in vanilla. Spoon batter over pecan mixture, gently smoothing top with an offset spatula.
Bake until a wooden pick inserted in center comes out clean, 35 to 40 minutes. Let stand 5 minutes. Place a flat serving plate over cake; carefully invert cake.
Recipe by Southern Cast Iron at https://southerncastiron.com/upside-down-praline-skillet-cake/