All-Blueberry Triple-Layer Cake 

Step 1: Dehydrated Blueberry Powder (Oven Method) 

1. Preheat oven to 170–180°F (75–80°C).

2. Wash 250 g blueberries, pat completely dry.

3. Slice berries in half and arrange cut-side up on a parchment-lined sheet pan.

4. Dry in oven for 4–6 hours, propping door slightly open.

5. Blueberries are done when fully brittle (no bend, no tackiness).

6. Cool completely, then grind to a fine powder.

7. Sift and re-grind any coarse pieces.

Yield: ~3–4 tbsp blueberry powder

Storage: Airtight container at room temperature

Step 2: Blueberry Reduction (Shared Base) 

1. Combine 500 g blueberries, 60 g granulated sugar, and 1 tbsp lemon juice in a saucepan.

2. Simmer over medium heat for 20–30 minutes, stirring frequently.

3. Reduce until thick and jammy.

4. Blend smooth and cool completely.

Reserve:

120 g for cake batter

1–2 tbsp for glaze

Extra may be kept as backup

Step 3: Blueberry Compote (Filling) 

1. Cook 200 g blueberries, 50 g granulated sugar, and 1½ tbsp lemon juice over medium heat until berries burst.

2. Stir 1½ tsp cornstarch with 1½ tbsp water.

3. Add slurry and cook until thickened.

4. Cool completely before using.

Step 4: Blueberry Cake Layers (3 × 6") 

1. Preheat oven to 350°F (175°C). Grease and line three 6-inch pans.

2. Toss 180 g blueberries with 1 tbsp all-purpose flour; set aside.

3. Whisk 240 g flour, 2½ tsp baking powder, ½ tsp baking soda, and ¾ tsp salt.

4. Cream 170 g (12 tbsp) unsalted butter with 300 g granulated sugar for 3–4 minutes until light and fluffy.

5. Beat in 3 large eggs, one at a time.

6. Whisk 120 g sour cream (½ cup), 120 g whole milk (½ cup), 120 g blueberry reduction, and 1 tsp vanilla extract (optional).

7. Alternate adding dry ingredients and wet mixture to the butter mixture, mixing just to combine.

8. Gently fold in blueberries.

9. Divide batter evenly between pans (≈520 g per pan).

10. Bake 30–45 minutes, until a toothpick comes out clean.

11. Cool 15 minutes, then turn out and cool completely.

Step 5: Blueberry Swiss Meringue Buttercream 

1. Combine 120 g egg whites (4 whites) and 240 g granulated sugar in a heatproof bowl.

2. Heat over simmering water, whisking constantly, until 160°F (71°C).

3. Transfer to mixer and whip until stiff, glossy peaks form and bowl is cool.

4. Add 340 g (24 tbsp) unsalted butter, one cube at a time.

5. Continue mixing until smooth and silky.

6. Beat in 1½–2 tbsp dehydrated blueberry powder and a pinch of salt.

7. Taste and adjust if needed.

Step 6: Assemble the Cake 

1. Level cake layers, if needed.

2. Place first layer on a cake board.

3. Pipe a buttercream dam using ~½ cup frosting.

4. Fill center with ½–⅔ cup blueberry compote.

5. Repeat with second layer.

6. Add final layer and frost entire cake.

7. Chill 20–30 minutes before glazing.

Step 7: Blueberry Glaze (Optional) 

1. Stir 40 g powdered sugar with 1–2 tbsp blueberry reduction until pourable.

2. Spoon gently onto the top center of the chilled cake.

Step 8: Decoration