Preheat your oven to 325°F (165°C) and grease a 13x9 inch glass baking pan.

Whisk together all-purpose flour, granulated sugar, salt, baking powder, and baking soda in a bowl until well combined.

In another bowl, combine buttermilk, eggs, egg yolk, vanilla extract, and vegetable oil; whisk until smooth.

Cut the unsalted butter into the dry ingredients until crumbly.

Pour the wet ingredients into the dry mixture in two parts, gently folding until just combined.

Pour the batter into the prepared pan and bake for approximately 45 minutes until golden and a toothpick inserted comes out clean.

Allow the cake to cool for about 15 minutes, then poke holes and prepare the glaze.

Melt brown sugar and butter in a saucepan, then whisk in confectioner’s sugar and pour over the cake.

Let the cake sit for 30 minutes to absorb the glaze before slicing and serving.