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 3-5 minutes. This incorporates air into the mixture, 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 step 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 up too much.

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 slightly longer baking times.

Once baked, remove the cake from the oven and allow it to cool in the pan for 10 minutes. Then, carefully transfer it to a cooling rack to cool completely.

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

Gradually add the sifted powdered sugar, mixing on low speed until combined.

Add the heavy cream and vanilla extract, then beat on high speed for 2-3 minutes until the frosting is light and fluffy. Adjust the cream as needed to achieve your desired consistency.

Once the cake is completely cool, spread the frosting evenly over the top and sides of the cake using a spatula.

For decoration, you can top the cake with additional fresh blackberries if desired.

Slice and serve your delicious high altitude blackberry cake!