Steps 1-3 are for making the lime curd. Use a zester or grater to zest 2 limes, careful not to get any of the white pith. Juice enough limes to get 1/2 cup of juice. My limes were small so I needed to use 5.
In a small bowl beat together 3 eggs plus one egg yolk. In a medium saucepan, off heat, stir together lime juice, lime zest, sugar, beaten eggs, and salt until smooth. Turn on heat to medium low and cook mixture until thickened stirring constantly (do not allow to come to a simmer and do not allow to sit or you chance scrambling the eggs). It is thick enough when it coats the spoon and a swipe of your finger leaves a stripe, as pictured. This takes bout 10 minutes.
Turn heat down to low. Stir in butter a couple cubes at a time until fully melted and incorporated. Transfer lime curd to a small bowl with a lid. Place a layer of plastic wrap directly on surface to prevent a skin from forming. Cover and refrigerate at least 4 hrs, up to overnight. Overnight is best.
Steps 4-6 are for making the cakes. Preheat oven to 350F. Grease and flour the only the bottoms of 2, 9 in cake pans. Place strawberries and water in blender. Blend into a smooth puree.
Add cake mix, eggs, oil, and strawberry puree to the bowl of a stand mixer. Mix on low until ingredients have just incorporated. Increase speed to medium and beat for 2 minutes.
Divide cake batter equally between cake pans. Firmly tap each pan on counter once to knock any bubbles in batter to the surface. Place cake pans in oven. Bake 30 minutes, rotating position halfway through baking. Remove from oven. Let cool completely in cake pans.
Steps 7 & 8 are for icing and cake assembly. Place butter in a medium microwave safe bowl. Heat in microwave until fully melted. Add powdered sugar and vanilla to butter. Wisk to combine. Add water 1 tbs at a time, wisking to incorporate after each addition. Add 2 tbs of lime curd, wisk until smooth.
Gently run a butter knife around the edges of each cake to loosen from the pan. Turn one cake out onto a large plate. Transfer to a large platter, bottom down. Spread lime curd evenly over top. Turn second cake out onto the large plate. Carefully lift cake up and place on top of lime curd layer. Drizzle icing over top. Place in fridge uncovered for 1 hour to set icing before serving.
