
Preheat oven to 325°F (160°C).
Spray a 14.5 cup bundt pan heavily with gluten-free cooking spray. I use coconut oil cooking spray.
To make the homemade buttermilk, add the vinegar to the cup of milk and stir.
In a large bowl, cream the softened butter and 2 cups sugar together with your mixer until fluffy.
Add the pure vanilla extract and eggs to the butter mixture and mix until smooth.
Add the baking powder, baking soda, and gluten-free flour and mix until fully combined.
Pour in the homemade buttermilk to the cake batter and mix until fully combined. The cake batter will be very thick.
Pour the cake batter into the greased bundt pan and bake on the middle rack for 70 minutes, or until a toothpick entered into the center comes out clean.
Melt the butter in a small saucepan over medium-low heat. Add in 3/4 cup granulated sugar, pure vanilla extract, and water. Bring to a very low boil, then reduce the heat and simmer for 1 minute until the sugar is dissolved.
Poke holes all over the warm cake with a butter knife (or cake tester, or chopstick) and pour the butter glaze evenly over the cake while still in the pan.
Allow the cake to cool completely in the pan and then invert the cake onto a serving plate.
Sprinkle with powdered sugar and enjoy!