Preheat your oven to 350°F (175°C) and grease a bundt pan with butter or nonstick spray.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition and then stir in the vanilla extract.
In a separate bowl, combine the all-purpose flour and baking powder; gradually add this to the creamed mixture alternately with the buttermilk.
Fold in the spinach puree gently until just combined, being careful not to overmix.
Pour half of the batter into the prepared bundt pan and smooth the top, then layer the remaining batter on top.
Bake in the preheated oven for 50-60 minutes or until a toothpick inserted in the center comes out clean.
Once baked, allow the cake to cool in the pan for about 15 minutes before inverting it onto a wire rack.
For the glaze, whisk together powdered sugar and milk until smooth; drizzle over the cooled cake.
Once the glaze sets, slice the cake and enjoy!