Preheat the oven to 350 degrees Fahrenheit and spray a 9x13 inch baking pan with pan spray.

In a medium bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.

In a large bowl, whisk together the buttermilk, milk, vanilla, oil, and eggs. Add the dry ingredients to the wet ingredients and whisk until combined.

Pour the batter into the prepared pan and bake for 35-40 minutes, until a toothpick comes out clean from the center of the cake.

Let the cake cool completely before inverting on a cooling rack. Carefully slice the cake through the center, so the top and bottom create two layers.

After the cream filling has been made, spread an even layer of filling between the two layers of cake, smoothing along the sides for clean edges.

Remove from the heat and add to a heat-proof bowl or quarter sheet pan. Cover with plastic wrap so the plastic touches the top of the mixture and prevents a film from forming. Chill in the fridge for 40 minutes

Add the room temperature butter to a stand mixer and whip with the whisk attachment for about 8 minutes, until the butter is light and fluffy.

Scrape down the sides and bottom of the bowl and switch to the paddle attachment. Add the vanilla and salt and beat on medium-high for 10-13 minutes, until smooth and fluffy.