Preheat the oven to 325 F (163 C). Grease a 10- inch (12 cup capacity bundt pan) with nonstick spray. Use a pastry brush to work the nonstick spray into all the grooves of the pan.
In a large bowl, whisk the flour, baking powder and salt together until well combined.
3 cups (360 g) all purpose flour, 1 teaspoon baking powder, ½ teaspoon salt
In the bowl of a stand mixer fitted with paddle attachment, beat the sugar and key lime zest together until a sand like consistency forms.
2 cups (400 g) granulated sugar, 4 key limes, zested
Beat in the butter and sugar together until light and fluffy. This takes about 2 - 3 minutes at medium speed.
1½ cups (339 g) unsalted butter, room temperature
On low beat in the eggs and vanilla extract one at a time, waiting until the previous ingredient is just combined before adding the next. Mix until well combined.
5 large eggs (US), room temperature, 1 teaspoon (5 ml) vanilla extract
By hand or on the lowest mixing speed, mix the dry ingredients into the wet alternating with the milk and key lime juice (in this order: dry, key lime juice, dry, milk, dry) until the batter is just combined.
¼ cup (59 ml) key lime juice, room temperature, 1 cup (237 ml) milk, room temperature
Evenly scoop the lime pound cake batter into the prepared bundt pan. Bake the pound cake bundt for 55 - 70 minutes or until a toothpick inserted into the center of the pound cake comes out with a few moist crumbs. Cool the cake in the loaf pan for 10 minutes. Place a serving plate or even another cooling rack on top of the bundt pan. Invert the pan to the top of the bundt cake is now on the bottom and lift up the pan. Cool the cake completely before topping with the key lime glaze. Make the key lime glaze by whisking the powdered sugar with the key lime juice and vanilla extract until well combined. If needed add more juice for a thinner glaze or more sugar for a thicker glaze.
1½ - 2 cups (180 - 240 g) powdered sugar, ½ teaspoon vanilla extract , 3 - 4 tablespoon (45 - 60 ml) key lime juice
Pour the glaze over the cooled pound cake. Zest a key lime on top of the wet glaze if you like. Allow 30-45 minutes for the glaze to set, slice and enjoy.