Preheat the oven to 350°F (175°C) and line a muffin tin with 12 cupcake liners.

In a medium bowl, whisk together the all-purpose flour, baking powder, and fine salt. Set aside.

In a large bowl, or the bowl of a stand mixer, cream the unsalted butter and sugar-free sweetener on medium-high speed for approximately 2 minutes, until light and fluffy. Add the eggs, vanilla extract, and almond extract (if using), and mix until fully incorporated.

Add the sour cream and mix on medium speed, scraping down the sides of the bowl as needed to ensure an even texture. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Avoid overmixing.

Divide the batter evenly among the cupcake liners. Bake in the preheated oven for 18–21 minutes, or until a toothpick inserted into the center comes out clean. Allow the cupcakes to cool in the pan for 10 minutes before transferring them to a wire rack to cool completely.

Fill a medium saucepan with approximately 1 inch of water and bring to a simmer. Ensure your mixing bowl is clean and grease-free by wiping it with vinegar or lemon juice.

In the clean mixing bowl, combine the egg whites and sugar-free sweetener. Place the bowl over the simmering water, ensuring the bottom does not touch the water. Whisk constantly for 3–5 minutes, or until the mixture reaches 160°F (71°C) and feels smooth to the touch.

Remove the bowl from the heat and dry its base. Using a stand mixer fitted with the whisk attachment, beat the mixture on medium-high speed for 5–8 minutes, or until stiff, glossy peaks form and the bowl is cool to the touch.

Reduce the mixer speed to medium and gradually add the room-temperature butter, one small piece at a time. Mix in the salt, vanilla extract, and almond extract. If desired, add gel food coloring. Beat on low speed for 2–3 minutes to achieve a thick, smooth consistency.

Transfer the frosting to a piping bag fitted with your desired tip (e.g., Wilton 1M) and decorate the cooled cupcakes as desired.