In small saucepan, combine lemon zest, lemon juice, egg yolks, sugar, corn starch and butter.
Cook on low heat until everything is combined.
Once everything is combined, increase the heat to medium. Stirring every 30 seconds, cook the mixture 12-15 minutes, or until it thickens.
Pour the curd into a heat proof bowl. Place a piece of plastic wrap over the lemon mixture. Press it down so it directly touches the curd.
Set aside and allow the curd to come to room temperature.
Once the curd is set, divide it into three portions. Two portions go in between the layers.
The third portion is spread at the top of the cake over the Lavender buttercream. Then add sprinkles.