Preheat the oven to 325°F. Thoroughly grease a 12-cup Bundt pan. Dip a paper towel into a generous amount of shortening and spread it around the pan, making sure to coat all its nooks and crannies. Lightly dust the greased pan with flour (using a sift works well for this). Tap and rotate the pan to create a thin, even coating of flour across the pan’s interior. Gently tap out any excess flour over the sink.
In a medium bowl, whisk together the flour, salt, and baking powder.
3 3/4 cups cake flour, 1 1/4 tsp. kosher salt, 1/2 tsp. baking powder
Add the softened brown butter, granulated sugar, and dark brown sugar to the bowl of a stand mixer fitted with the paddle attachment.
2 cups granulated sugar, 3/4 cup dark brown sugar
Cream on medium-high speed for 4-5 minutes, scraping down the bowl occasionally, until noticeably lighter and fluffy. Don't rush this step. Start Timer
Add the eggs one at a time, beating until each is incorporated before adding the next. Scrape down the bowl as needed.
6 large eggs
Mix in the maple syrup, vanilla, and maple extract (if using)
1/2 cup pure maple syrup, 1 Tbsp. vanilla bean paste, 1 tsp. maple extract
Whisk the milk and sour cream together.
2/3 cup whole milk, 1/2 cup full-fat sour cream
With the mixer on low, add the flour mixture in three additions, alternating with the milk mixture in two additions, beginning and ending with the flour. Mix only until combined.
Finish folding the batter gently by hand with a rubber spatula, making sure there are no pockets of flour at the bottom of the bowl.
Spoon the batter evenly into the prepared Bundt pan and tap the cake on the counter a few times.
Bake for approximately ~60-70 minutes, beginning to check around 60 minutes. Start Timer The cake is finished when a long wooden skewer inserted into the thickest portion comes out clean or with a few moist crumbs.
Cool in the pan for 15 minutes. Start Timer