Prepare the Oven and Pan: Preheat the oven to 350°F (175°C). Line a 12-cup cupcake pan with cupcake liners and set aside.

Combine the Dry Ingredients: In a large mixing bowl, whisk together the cake flour, almond flour, baking powder, and kosher salt until evenly combined.

Mix the Butter and Sugars: In a separate bowl, whisk the room-temperature brown butter with the light brown sugar and granulated sugar until the mixture is smooth.

Add the Eggs and Extracts: Whisk the eggs into the butter-sugar mixture, followed by the vanilla extract and almond extract. Continue mixing until everything is thoroughly incorporated.

Combine the Batter: Stir approximately half of the dry ingredient mixture into the wet ingredients. Add the whole milk, then stir in the remaining dry ingredients. Mix until the batter is fully combined, taking care not to leave visible pockets of flour.

Fill the Cupcake Liners: Divide the batter evenly among the 12 prepared liners, using approximately ¼ cup of batter per liner. The liners should be about half full.

Bake the Cupcakes: Bake at 350°F (175°C) for 18–22 minutes, or until a cake tester inserted into a cupcake comes out clean.

Cool the Cupcakes: Remove the cupcakes from the pan and transfer them to a wire rack. Allow them to cool completely before adding the frosting.

Cream the Butter and Sugar: Place the room-temperature brown butter and powdered sugar in a large mixing bowl. Using a handheld mixer or a stand mixer fitted with a paddle attachment, beat until the mixture becomes smooth.

Add the Flavorings: Add the almond extract, vanilla extract, and a pinch of kosher salt. Beat on high speed for 4–5 minutes, until the frosting is light and fluffy.

Adjust the Consistency: If the frosting is too thick, add the heavy cream one tablespoon at a time, mixing after each addition until the desired consistency is reached. Use 2–3 tablespoons as needed.

Frost the Cupcakes: Once the cupcakes have cooled completely, spread or pipe the brown butter frosting over their tops.

Add the Almond Garnish: Finish the frosted cupcakes with sliced toasted almonds, if desired, for an extra nutty flavor and a delicate crunch.

Melt the Butter: Place the butter in a small saucepan over medium heat and allow it to melt completely.

Brown the Butter: Stir frequently as the butter cooks. Watch for brown specks forming at the bottom of the pan and notice the development of a nutty aroma.

Stop the Cooking: Once the brown specks appear, stir for just a few more seconds, as the butter can brown very quickly and burn easily.

Transfer and Cool: Immediately pour the browned butter into a different bowl to prevent residual heat from burning it. Refrigerate for at least 20 minutes to cool before use, then ensure it reaches room temperature for the recipe.

Account for Evaporation: Browning ½ cup (1 stick or 113 g) of butter typically reduces its weight by approximately 1 tablespoon (14 g) because of evaporation. Add an extra tablespoon of butter before browning, or replace the lost weight with regular unsalted butter if necessary when preparing larger batches.

Optional Alternative: If you prefer not to brown the butter, the source notes that melted butter can be used instead, although the characteristic toasted, nutty flavor will differ.