Preheat oven to 160°C (320°F). Grease a 12-cup round bundt cake tin with melted butter, then dust with flour. Tap the pan upside down to remove any excess flour.
Dry ingredients - Place flour, baking powder, bi-carb soda and salt in a large bowl, then whisk well to combine. Wet ingredients - In a separate small bowl, whisk together the milk and vanilla extract.
Using a stand or hand mixer, beat butter and sugar together using the paddle attachment on medium-high speed for 3 minutes (scrape the bottom and sides of the bowl halfway) or until pale and fluffy.
Decrease the speed to low and add in the eggs, but one at a time.
Still, on low speed, add in HALF of the dry ingredients, then pour in HALF of the wet ingredients. Once combined, add in the remaining dry and wet ingredients and beat until just combined. Don't over-mix.
Place half the batter into the prepared tin and tap it against the kitchen bench a few times, then add in the remaining batter. Smooth and level the surface using a spatula. Bake for 50 to 60 minutes or until a skewer comes out clean when inserted into the middle of the cake.
For the butter bourbon syrup - Once the cake is out of the oven, prepare the syrup. Place butter, sugar and water into a small saucepan over medium heat and bring to a simmer. Reduce heat to low and add in the bourbon and vanilla and simmer for 5 minutes.
Whilst the syrup is simmering, use a skewer to poke several holes in the cake. Pour the syrup evenly over the cake and let it stand for 10 to 15 minutes before removing the cake from the tin.