Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan to prevent sticking.

In a large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer on medium speed until light and fluffy, about 3-4 minutes. This incorporates air into the batter, helping the cake rise.

Add the eggs one at a time, mixing well after each addition. This ensures even distribution of the eggs throughout the batter.

Stir in the buttermilk and vanilla extract until just combined.

In a separate bowl, whisk together the sifted flour, baking powder, baking soda, and salt. This helps to evenly distribute the leavening agents.

Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Be careful not to overmix, as this can lead to a dense cake.

Gently fold in the blackberries using a rubber spatula, being careful not to break them apart.

Pour the batter into the prepared cake pan, smoothing the top with a spatula.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean. High altitude baking may require slight adjustments in time.

Once baked, remove from the oven and allow to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

In a mixing bowl, beat the softened butter with an electric mixer until creamy.

Gradually add the sifted powdered sugar, mixing on low speed until combined. This prevents a sugar cloud!

Add the heavy cream and vanilla extract, then beat on medium speed for 2-3 minutes until fluffy and smooth. Adjust the cream for desired consistency.

Once the cake is completely cool, place it on a serving plate. Spread a layer of frosting on top, then place the second layer (if making a two-layer cake) on top and frost the sides and top as desired.

Decorate with additional fresh blackberries if desired.