Preheat the oven to 350℉ (175 ℃). Grease a 9×9in pan and line it with parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Then set aside the flour mixture.
2 1/4 cups (281 g) all-purpose flour, spooned and leveled, 1 1/2 tsp baking powder, 1/4 tsp baking soda, 1/2 tsp salt
Add the softened butter and granulated white sugar to a large bowl. Cream together for 2 minutes with an electric mixer.(You can also use the bowl of a stand mixer fit with a paddle attachment.) 10 tbsp (140 g) unsalted butter, softened, 1 1/2 cups (300 g) granulated white sugar
Then add in the eggs and vanilla and mix on medium speed until pale in color and smooth, about 1 minute.
3 large eggs, at room temperature, 1 1/2 tbsp vanilla bean paste
Add the dry ingredients and buttermilk to the wet ingredients a little at a time, mixing on low speed until the cake batter is combined. Scrape the sides of the bowl with a rubber spatula as necessary.
1 cup (240 ml) buttermilk, at room temperature
Pour the batter into the prepared pan. Bake the cake for 39-44 minutes or until a toothpick comes out clean from the center.
Let the cake cool in the pan on a wire rack for 30 minutes. Carefully pull up on the parchment paper to release the cake from the pan, then continue to cool it on the wire rack until completely cooled.