Prepare the Blackberry Puree: In a small saucepan, combine fresh blackberries and lemon juice over medium heat. Simmer for about 10 minutes or until the mixture thickens, stirring occasionally. Once thickened, strain the puree through a fine mesh sieve to remove seeds, and let it cool completely while you prepare the cupcakes.

Preheat the Oven: Preheat your oven to 350°F (175°C). Line a standard cupcake pan with liners, making sure they sit snugly.

Combine Dry Ingredients: In a large mixing bowl, whisk together all-purpose flour, unsweetened dark cocoa powder, baking soda, baking powder, and salt.

Mix Wet Ingredients: In a separate bowl, beat together large eggs, granulated sugar, brown sugar, vegetable oil, and vanilla extract for about 2-3 minutes until smooth.

Combine Mixtures: Gradually add the buttermilk to the wet ingredients and mix until combined. Slowly incorporate the dry ingredients into the wet mixture, alternating with hot coffee or hot water.

Fill the Cupcake Liners: Carefully fill each cupcake liner about two-thirds full with the batter.

Bake the Cupcakes: Place the cupcake pan in the preheated oven and bake for 18-20 minutes or until a toothpick inserted in the center comes out clean.

Cool Completely: Once baked, remove the cupcakes from the oven and let them cool in the pan for about 5 minutes, then transfer them to a wire rack.

Prepare the Frosting: In a mixing bowl, beat softened unsalted butter on medium speed until creamy. Gradually add powdered sugar and cooled blackberry puree while mixing.

Frost the Cupcakes: Once the Dark Chocolate Blackberry Cupcakes are completely cooled, use a piping bag or knife to frost each cupcake with the blackberry frosting.