Place the frozen blueberries, granulated sugar, and water in a small saucepan and stir until combined.
Cover the saucepan and bring the berry mixture to a gentle simmer over medium heat.
Remove the lid and continue simmering for approximately 10–15 minutes, stirring from time to time. Cook until most of the liquid has evaporated and the blueberries have reduced to a thick, jam-like compote.
Transfer the finished compote to a small bowl and allow it to cool completely before using it in the cake.
Preheat the oven to 350°F (175°C). Place parchment-paper circles on the bottoms of three 6-inch round cake pans, leaving the sides of the pans unlined.
Separate the eggs, placing the whites in one large mixing bowl and the yolks in another.
Add the cream of tartar to the egg whites. Using a hand mixer on low speed, beat until the whites become foamy.
Add the 120 g granulated sugar gradually, approximately one spoonful at a time, while continuing to beat the egg whites. Whip until firm, stiff peaks develop, then set the meringue aside.
Add the milk, neutral oil, and vanilla extract to the bowl containing the egg yolks. Mix until evenly incorporated.
Sift the cake flour and baking powder directly into the yolk mixture. Stir until a smooth batter forms.
Gently fold approximately one-third of the whipped egg whites into the yolk batter. Fold in the remaining egg whites, taking care to stop as soon as the mixture is evenly combined.
Distribute the batter equally among the three prepared pans, using approximately 185–200 g of batter for each pan.
Tap the pans gently to release larger trapped air bubbles.
Bake for 25–27 minutes, or until the cake layers are lightly golden.
Immediately invert the baked cakes onto a wire rack and leave them upside down until completely cool. Once cooled, release the cakes from the pans and peel away the parchment paper.
After both the cake layers and blueberry compote have cooled completely, place the whipping cream, confectioners’ sugar, 3 tablespoons of the prepared compote, and vanilla extract in a large mixing bowl.
Beat with a hand mixer until the cream reaches stiff peaks.
Set the first cooled cake layer on a cake turntable.
Cover the surface with a generous amount of blueberry whipped cream, then spoon and spread some of the blueberry compote into the center of the cream layer.
Position the second cake layer on top. Repeat the same arrangement of blueberry whipped cream and compote.
Place the third and final cake layer over the filling to complete the stack.
For greater stability during the final coating, the stacked cake may be chilled briefly before proceeding.
Spread a thin layer of blueberry whipped cream over the entire stacked cake to form a crumb coat and trap loose crumbs.
If desired, refrigerate the cake for approximately 10 minutes. The whipped-cream coating will not become firm in the same manner as buttercream, but brief chilling can help stabilize the layers.
Apply a final, even coating of blueberry whipped cream over the top and sides of the cake.
Smooth the surface with a spatula or bench scraper, removing excess cream as necessary to create an even finish.
Arrange fresh or fully defrosted blueberries over the finished cake and add thyme sprigs as an optional garnish.
For additional color and presentation, a light dusting of confectioners’ sugar or a more abundant arrangement of blueberries may also be added.