In a mixing bowl, combine the icing sugar, cold cubed butter, vanilla extract, and salt. Use a stand mixer or hand mixer to beat the mixture on medium speed for about 2 minutes until it forms a smooth paste. Don’t forget to scrape down the sides of the bowl after the first minute.

Next, add in the egg yolks and mix until they are fully incorporated into the paste.

Gradually incorporate the flour, mixing on low speed until just combined. The mixture should come together to form a soft dough, but don’t overmix it.

Transfer the dough onto lightly floured parchment paper, placing another sheet of parchment on top. Roll out the dough to about 1/8 inch (3 mm) thickness before chilling in the fridge for at least 30 minutes.

For the chocolate batch, repeat the steps above, mixing the cocoa powder with the icing sugar at the beginning to ensure it’s evenly distributed.

Using a ruler and sharp knife, cut about 1/4-inch strips out of each batch of dough. Carefully lift these strips and alternate the colors, layering them side by side on a new sheet of parchment.

Once arranged, place another sheet of parchment on top and roll out to seal the layers together. Chill again for 30 minutes.

Preheat your oven to 320°F (160°C). After the dough has chilled, remove one layer of parchment. Lightly flour it, flip the dough over, and peel off the next sheet allowing for a little more flour dusting to prevent sticking.

Use a 4.5-inch round tart ring to cut out circles of the dough, and repeat this process for the second batch for the sides.

Carefully take the strips for the tart rings and press them against the sides of the ring, molding the bottom gently. Trim any excess pastry hanging over the top.

Place all tart shells into the freezer for about 30 minutes to 1 hour.

Bake the tart shells in the preheated oven for around 15 minutes or until the shells are lightly golden. Allow them to cool completely.

For the ganache, melt the chocolate and heated cream together over low heat. Stir until smooth and set aside to cool slightly.

Whisk the egg whites, sugar, and a pinch of salt together over simmering water until the mixture is warm to the touch and no longer gritty. Transfer to a stand mixer and whip until you achieve glossy peaks.

Switch to a paddle attachment and gradually add in the softened butter in tablespoon-sized pieces. Allow each addition to fully incorporate before adding the next. Finally, mix in the vanilla extract.

Set aside 2/3 of the white Swiss meringue buttercream for the topping. Melt semi-sweet chocolate, cool it, and then incorporate it into the remaining buttercream.

For the ridged top, pipe strips of the white buttercream into tight lines on parchment paper. Freeze for 30 minutes to set.

When the tarts have cooled, pipe chocolate ganache into each shell, followed by a layer of the chocolate Swiss meringue buttercream. Gently place the frozen ridged tops on each tart.