Add the icing sugar, meringue powder, water and lemon juice to a stand mixer and whisk for 10-15 mins until it is very thick. If it looks too dry just add a splash more water. Add the vanilla towards the end of the timer.
Split the mixture into 5 bowls and add food colouring to 4 bowls, leaving one white.
Place the mixtures into 5 piping bags fitted with a #1 or #2 piping nozzles.
Pipe even straight lines along a silicon mat and then allow them to dry for 3 hours or overnight.
Once dried, roll your hands along the top of them to break them up.
Beat the oil, butter and sugar together for 5m until light and fluffy. Add the eggs one at a time, scraping down the bowl in between. Sift the dry ingredients together and set to one side. Combine the wet ingredients together.
Add the dry ingredients to the mixer, alternating with the wet. You want to have 3 parts dry, two parts wet (dry, wet, dry, wet, dry).
Just before adding the batter into your cake tin, gently fold in a few tbsp of the sprinkles, being careful not to overmix them.
Place the batter into a lined brownie tin or sheet pan and bake for 30-35m at 180C or until a toothpick comes out clean.
Once baked remove from the tin immediately and allow to cool.
Mix egg whites with sugar in a ban marie and put it over a saucepan with simmering water for a few minutes until sugar dissolves or until it reaches 65°C on a digital thermometer.
Using a stand mixer, whisk the egg white mixture for about 5-7 minutes or until the meringue is firm and bowl feels cool to the touch.
Once the meringue is cooled, start adding soften butter slowly (about a Tbsp at a time) until it is fully incorporated. Add the vanilla and whisk for about another 2 minutes at medium-high speed until it is smooth.
Place the cooled cake upside down so the flat side is now at the top.
Spread a generous portion of frosting on top and swirl it to the edges with an offset spatula.
Finish by decorating with a handful of homemade sprinkles over the top.