For cake: Line a 9-inch round cake pan with parchment paper. Grease with 2 tablespoons butter; sprinkle with 2 tablespoons sugar, turning to coat. Place blackberries in prepared pan.
In a large bowl, beat remaining ½ cup butter and remaining ¾ cup sugar with a mixer at medium-high speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add egg and vanilla, beating well.
In a medium bowl, stir together flour, baking powder, baking soda, and salt. Reduce mixer speed to low. Gradually add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating just until combined after each addition. Beat in zest.
Spoon batter over blackberries in pan, smoothing with an offset spatula. Tap pan on counter twice to release air bubbles.
Bake until a wooden pick inserted in center comes out clean, 35 to 40 minutes. Let cool in pan for 10 minutes. Invert onto a wire rack, and let cool completely. Remove parchment paper.
For topping: In a medium bowl, beat cream, buttermilk, and confectioners’ sugar with a mixer at high speed until stiff peaks form. Dollop whipped cream over cake. Garnish with Blackberry Jam, if desired.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/blackberry-buttermilk-skillet-cake/