For the Cake:

1. Preheat and prepare: Preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan, or line with parchment paper for easier removal. Set aside.
2. Mix dry ingredients: In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until well combined. This ensures even distribution of leavening agents throughout the cake.
3. Combine wet ingredients: In a separate medium bowl, whisk together the buttermilk, vegetable oil, eggs, and vanilla extract until smooth and emulsified.
4. Create the batter: Pour the wet ingredients into the dry ingredients and stir with a wooden spoon or rubber spatula until just combined. Don’t overmix – a few small lumps are perfectly fine.
5. Add hot water: Carefully pour in the hot water and stir gently until fully incorporated. The batter will be quite thin – this is exactly what you want for a moist cake! Don’t be alarmed by the consistency.
6. Bake: Pour the batter into your prepared pan and tap gently on the counter to release any air bubbles. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean or with just a few moist crumbs. The cake should spring back lightly when touched.
7. Cool completely: Remove from the oven and let the cake cool in the pan on a wire rack for at least 1 hour before frosting. This is crucial – frosting a warm cake will result in melted, sliding frosting.

For the Buttercream Frosting:

1. Beat the butter: In a large bowl using an electric mixer, beat the softened butter on medium speed for 2-3 minutes until light and fluffy.
2. Add sugar gradually: With the mixer on low speed, gradually add the powdered sugar, one cup at a time, beating well after each addition.
3. Incorporate liquids: Add the vanilla extract, salt, and 2 tablespoons of heavy cream. Beat on medium-high speed for 3-4 minutes until light and fluffy.
4. Adjust consistency: If the frosting is too thick, add more cream one tablespoon at a time. If too thin, add more powdered sugar ¼ cup at a time until you reach your desired spreading consistency.
5. Frost the cake: Once the cake is completely cool, spread the frosting evenly over the top using an offset spatula or butter knife. Create swirls or keep it smooth – whatever suits your style!
6. Set and serve: Let the frosted cake sit for 15-20 minutes before slicing to allow the frosting to set slightly. Cut into squares and serve.