Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a large bowl, whisk together flour, baking powder, baking soda, and salt.

In a separate bowl, cream together butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in vanilla.

Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.

Pour batter into the prepared pan and bake for 45-50 minutes, or until a wooden skewer inserted into the center comes out clean.

While the cake is baking, make the glaze. In a saucepan, combine butter, sugar, and water. Bring to a boil, stirring constantly.

Remove from heat and stir in vanilla extract.

As soon as the cake comes out of the oven, poke holes all over the top with a skewer or toothpick. Pour the warm glaze over the cake.

Let the cake cool completely in the pan before serving.