Preheat oven to 350F. Grease and flour two 9-inch round cake pans; set aside. In a medium bowl, whisk together flour, baking powder, baking soda, and salt; set aside. 

Using an electric mixer, cream together butter and sugar until light and fluffy, 3-5 minutes. Scrape down sides of bowl and beat in eggs, one at a time, beating 1 minute between each. Beat in vanilla. Add dry ingredients all at once and beat on low speed until just combined. Add buttermilk and beat on low 1 minute, then increase speed to medium and beat 3 additional minutes.

Divide batter evenly between prepared pans. Bake 30-35 minutes, or until a toothpick inserted comes out with just a few crumbs attached. Remove from oven and cool 10 minutes in pans, then turn out onto wire racks and cool completely before frosting.

Chocolate Frosting With a bowl or hand-held mixer, blend butter, powdered sugar, cocoa powdered, and salt until smooth, about 30 seconds. Add corn syrup and vanilla and mix until just combined, 5-10 seconds. Scrape sides of bowl, then add melted chocolate and pulse until smooth and creamy, 10-15 seconds.

Assemble Cake Place one cake layer on a serving platter and spread 1 cup of  frosting evenly over surface. Place second cake layer on top. Use remaining frosting to frost top and sides of cake. The cake can be kept at room temperature for up to 1 day before serving. Once cut, leftovers should be stored in an airtight container at room temperature for up to 4 days.