Preheat oven to 350°F and line three 6" cake pans with parchment paper.

In a small pot, heat milk until just before it simmers. Stir in the earl grey tea bags, cover, and steep for 5-10 minutes. Use a spoon to press tea bags against the side of the pot to squeeze out all the tea-infused liquid inside before discarding. Reserve 3-4 tablespoons of this earl grey infused milk in a separate small bowl for the frosting later. Set aside to cool.

In a medium bowl, stir together cake flour, baking powder, and salt. Set aside.

In a large mixing bowl, use an electric mixer to beat together melted butter, oil, and sugar. Beat in the eggs and vanilla extract until all combined.

With the mixer on low speed, alternate adding in half the flour mixture, then half the earl grey infused milk at a time, until both are just combined. Don't overmix.

Divide the batter evenly into lined cake pans. For 6" cake pans, I filled each with about 450g of batter.

Bake cakes for 30-35 minutes or until a toothpick inserted in the middle comes out clean. Let them cool in their pans for 10 minutes before flipping the cakes out upside down and letting them cool completely.

In a large mixing bowl, beat the softened butter with an electric mixer and paddle attachment until it becomes creamy and light in colour, about 4 minutes.

With the mixer on low speed, add in all the powdered sugar. As the sugar is combined, turn up the mixer to medium speed.

Add in the earl grey infused milk one tablespoon at a time. Beat the frosting until it becomes fluffy and spreadable, about 5 minutes. 

Divide a small portion of icing into a piping bag and snip off the tip of the bag. If you're doing additional icing colours, separate small portions of icing out into small bowls and stir in gel food colouring and transfer to their own small piping bags.

Level your cake layers if needed until they are flat on both sides. Lay down your first layer on a turntable. Use an offset spatula to smooth a layer of buttercream on top of the cake layer.

With the buttercream you portioned out into a piping bag, pipe a thick ring of frosting on the edge of the cake. This will act as a wall or barrier. Spread 2-3 heaping spoons of lemon curd into the middle of the ring and smooth out.

Place your second layer on top and repeat frosting an even layer, piping a ring of buttercream, and filling the middle with lemon curd. Top with your last layer.

Smooth buttercream around all sides and top of the cake and smooth out. Decorate with any designs you wish.