1-First step: Prep the pan and oven Preheat the oven to 350°F. Lightly grease a 9-inch square pan, line the bottom with parchment paper, and grease the parchment too. This helps the cake release cleanly after baking. If you are baking ahead for a party or picnic, this step also saves time later.

2-Second step: Mix the cake batter Beat the softened butter and granulated sugar together until the mixture looks light and fluffy. Add the eggs one at a time so they blend in smoothly, then mix in the vanilla extract and buttermilk. In a separate bowl, whisk together the cake flour, baking powder, baking soda, and salt. Add the dry ingredients to the wet mixture and stir just until combined. Avoid overmixing so the cake stays tender.

3-Third step: Bake the cake layer Spread the batter evenly into the prepared pan. Bake for 20 to 25 minutes, checking early if your oven runs hot. The cake is ready when a toothpick inserted in the center comes out with a few moist crumbs. Do not wait for the toothpick to come out completely clean, or the cake may turn dry.

4-Fourth step: Cool the cake completely Let the cake cool in the pan for 10 minutes, then invert it onto a wire rack and let it cool fully. This helps keep the crumb soft and prevents the mousse from melting when you assemble the dessert. You can also bake this cake layer a day ahead and wrap it well once cool.

5-Fifth step: Make the blueberry mixture Puree the fresh blueberries until smooth. Bloom the unflavored gelatin in the cold water and set it aside. Place the blueberry puree and sugar in a saucepan over medium heat and bring it to a gentle simmer. Cook for 15 to 20 minutes, stirring often, until the mixture reduces slightly. It may still look loose at this stage, and that is normal.

6-Sixth step: Add the gelatin and chill the mixture Stir the bloomed gelatin into the hot blueberry mixture until it fully dissolves. Then let it cool completely in the fridge. This is the part that gives the mousse structure later. If the mixture looks thin, that is fine. Once it is folded into whipped cream and chilled, it will set properly.

7-Seventh step: Whip the cream In a chilled bowl, whip the heavy cream with the confectioners’ sugar and vanilla extract until stiff peaks form. Keep an eye on the texture so you do not overwhip. You want the cream to be firm but still smooth.

8-Eighth step: Fold everything together Fold half of the cooled blueberry mixture into the whipped cream gently, then fold in the rest. Use a light hand so you do not knock out the air. This is what gives Blueberry Mousse Cake its airy, cloudlike texture.

9-Ninth step: Assemble and chill Line a clean 9-inch square pan with parchment overhang so the dessert lifts out easily later. Place the cooled cake inside, spread the mousse evenly over the top, and smooth the surface. Refrigerate for at least 4 hours, or until the mousse is fully set.

10-Final step: Slice and garnish Lift the cake out using the parchment overhang. Slice with a sharp knife, wiping the blade between cuts for neat edges. Finish with fresh blueberries on top for a simple, pretty garnish.