Preheat oven to 350*F
Line a 9x13 pan with parchment on the base and spray sides with cooking spray.
In a measuring cup mix soy milk and vinegar and let stand until curdled, about 5 mins.
In a small bowl, combine warm water and milled flax and let stand to thicken, about 5 mins.
In a mixer with paddle or whisk attachment, cream butter, sugar and vanilla together until fluffy.
Add flax egg and mix until combined.
In another bowl, combine cake flour and baking powder.
Alternate the addition of dry ingredients and milk into butter mixture and mix on med/low until well combined, stopping to scrape sides to ensure batter is well mixed.
Pour batter immediately into pan and bake for about 25 minutes or until golden brown. Allow to sit in pan until cool to the touch, then cover with saran to retain moisture. (Cake can be frozen after cooled completely, simply wrap in 2 layers of saran wrap and freeze)
If decorating a sheet cake, feel free to leave the cake in the pan and pipe directly onto the top of the cake.
Cream butter and shortening with a whisk attachment until completely smooth.
Add in icing sugar and vanilla extract, stirring in until mixed, then whip until fluffy.
Add plant milk if icing is too thick.
To decorate a 9x13 sheet cake with 2 colors, portion out about 2 cups of icing and color to desired color.
Place icings into piping bags fitted with two different piping tips (I used a 1M and 6B) and begin piping with one color- rosettes and stars all around the cake.
Fill in the spaces with the other color and continue to alternate colors until the whole cake top is covered in icing.
Refrigerate any remaining buttercream in an air tight container for up to 3 weeks.
Cake can be refrigerated for up to 1 week decorated and up to 3 days covered at room temperature.