Preheat oven to 350°F. Line a muffin tin with 12 cupcake liners.

Place 2 and 1/4 teaspoons lemon juice in a measuring cup. Add enough milk to the measuring cup to equal 3/4 cup; let sit for a few minutes.

In a medium-sized mixing bowl, whisk together the flour, baking soda, cinnamon, and salt; set aside.

In a stand mixer fitted with the paddle attachment, beat the butter and both sugars together until combined. Add in the egg, egg yolk, and vanilla. Mix on high until light and fluffy, about 1-2 minutes. Stir in the mashed sweet potato and remaining 1 and 1/2 teaspoons lemon juice. Batter may be lumpy.

With mixer on low, alternate adding the dry ingredients with the milk mixture until just combined. Do not overmix. Evenly distribute batter among cupcake liners, then bake for 18-22 minutes, or until a toothpick inserted in the center comes out with few crumbs attached. Allow cupcakes to cool for 15 minutes before transferring to a wire rack to cool completely.

In a medium saucepan, melt the salted butter over medium-low heat. As the butter melts, it’ll start to foam, then turn a clear golden color. Continue to stir the butter, scraping up any browned specks from the bottom of the pan as it starts to brown. It should have a nutty aroma. Once browned and nutty, pour into a shallow bowl, then place into the freezer until solid, about 30 minutes.

In the bowl of a stand mixer fitted with the paddle attachment, cream the solidified browned butter with the unsalted butter until creamy. Add the powdered sugar, a cup at a time, until combined. Add the maple syrup, milk, and vanilla and continue mixing until fluffy.

Pipe or spread Buttercream onto cooled cupcakes. Top with sprinkles and enjoy!