Preheat oven to 350 degrees and line 18 muffin cups with paper liners.
In a large bowl, beat butter and sugar using an electric mixer until creamy, about 3 minutes. Add eggs one at a time, scraping down sides of bowl after each addition. Add vanilla and beat to combine.
In a medium bowl, whisk together flour, baking powder, baking soda and salt.
Add flour mixture to butter mixture in thirds, alternating with buttermilk, beginning and ending with flour. Beat just until smooth.
Gently fold in ¼ cup of Blackberry Sauce to make a swirl. Divide mixture evenly between muffin cups.
Bake until wooden pick inserted in center of cupcake comes out clean, about 25 minutes. Let cupcakes cool in pans for 10 minutes, then remove to wire racks to finish cooling.
Make Blackberry Buttercream. In a large bowl, beat butter at medium speed until smooth and creamy.
Add blackberry sauce and 1 cup of sugar, and beat until mixed well. Add remaining sugar 1 cup at a time and beat until thick and fluffy.
Spread Blackberry Buttercream on cupcakes (use a pastry bag if desired). Top with a blackberry. Store in refrigerator.