Preheat the oven to 350°F / 175°C and line a cupcake pan. Set aside.

In a large bowl or the bowl of a stand mixer, cream together 6 Tbsp butter and 3/4 cup sugar until the mixture becomes lighter in color (1-2 minutes).

Mix in 3 egg whites on a medium speed, scraping down the sides and bottom of the bowl as needed with a rubber spatula.

Mix in 1/3 cup sour cream, 1/3 cup whole milk, 1/4 cup lime juice, 1 tsp vanilla, 2 tsp lime zest and a squirt of gel food coloring on a medium speed until combined.

In a separate bowl, whisk together 1 1/4 cups flour, 2 tsp baking powder and 1/4 tsp salt.

Mix in the dry ingredients in two additions, mixing on a low speed until just combined. Be careful not to over-mix the batter!

Fill the cupcake liners 3/4 full and bake for 15-17 minutes or until a toothpick comes out with a few moist crumbs.

Remove cupcakes from oven and let them to cool for 2-3 minutes in the pan, then place them on a wire rack to finish cooling.

While the cupcakes bake and cool, make the lime curd filling.

Whisk together 3 large eggs, 3/4 cup sugar, 1/4 tsp salt, 1/2 cup lime juice and 1 Tbsp lime zest in a medium saucepan until smooth.

Heat over a medium low head for 10-15 minutes stirring constantly until the mixture thickens.

Turn off the heat and stir in 6 Tbsp of butter and a squirt of electric green gel food coloring until smooth and evenly colored.

Pour into a separate bowl to cool and set aside.

As the lime curd filling cools, make the lime buttercream.

Beat 1 cup of room-temperature butter on a medium-high speed for 2 minutes with a whisk attachment until the butter becomes lighter in color.

Mix in 2 Tbsp lime juice, 1 tsp lime zest, 1 tsp vanilla extract, 1/2 tsp salt, and a generous squirt of electric green food coloring on a low speed.

Slowly mix in 3 1/2 cups of powdered sugar, 1 cup at a time on a low speed.

Mix on low for a couple minutes until the ingredients are fully incorporated and the desired consistency is reached. 

If the frosting is too thick, add in additional lime juice (1 Tablespoon at a time). If the frosting is too thin, add in more powdered sugar (1/4 cup at a time).

Use a small knife or 1 inch circle cutter to remove the center from each cooled cupcake.

Fill each cupcake with a generous spoonful of lime curd.

Pipe a large swirl of lime buttercream on top of each cupcake using a large round piping tip. Chill the cupcakes in the fridge for 10 minutes or the freezer for 5 minutes to help the frosting firm up.

While the cupcakes chill use black, white, yellow, and red fondant to make Slimer's mouth and eyes.

Remove the cupcakes from the fridge and add a Slimer face to each cupcake, then enjoy!