Start by heating your oven to 350°F (175°C). Prepare two 9-inches round cake pans by greasing them with butter or oil and lightly dusting with flour, or line them with parchment paper for easy removal later.

In a large mixing bowl, whisk together the flour, baking powder, and salt. Set this mixture aside for now.

In a separate bowl, combine the sugar, eggs, and vegetable oil. Beat them together until the mixture is smooth. Then add the milk and vanilla extract, mixing until fully incorporated.

Slowly add the dry ingredients to the wet ingredients, stirring gently until everything is just combined. Be careful not to over mix, as this can make the cake dense instead of light and fluffy.

Divide the batter evenly between the two prepared cake pans. Bake in the preheated oven for 35–40 minutes. To check if it’s done, insert a toothpick into the center; it should come out clean. Let the cakes cool in the pans for about 10 minutes, and then transfer them to a wire rack to cool completely.

In a small saucepan, combine the blueberries, water, sugar, and cornstarch. Cook over medium heat, stirring constantly, until the mixture thickens. Remove from heat, stir in the lemon juice, and allow the filling to cool to room temperature.

Using an electric mixer beat the butter until it’s creamy. Gradually add the heavy cream, vanilla extract, and a pinch of salt. Continue to beat until the butter cream is light, fluffy, and smooth.

Place one cake layer on your serving plate. Spread the cooled blueberry filling evenly over the top. Add the second cake layer on top, and then cover the top and sides with the vanilla buttercream. Decorate as you like. Chill the assembled cake for 15–20 minutes to let the frosting set before slicing.