Make the lemon curd: In a heatproof bowl set over simmering water, whisk together 1/4 cup lemon juice, 2 teaspoons lemon zest, 1/3 cup sugar, and 4 egg yolks. Whisk constantly for 8–10 minutes until thickened and coats the back of a spoon.

Finish the curd: Remove from heat and whisk in 3 tablespoons cubed butter until melted and glossy. Pour into a bowl, press plastic wrap onto surface, and refrigerate at least 2 hours.

Prepare cake pans: Preheat oven to 350°F (176°C). Grease three 8-inch round cake pans and line bottoms with parchment paper.

Mix dry ingredients: In a medium bowl, whisk together 2 1/2 cups flour, 1 3/4 teaspoons baking powder, 1/4 teaspoon baking soda, and 1/2 teaspoon salt.

Cream butter and sugar: In a stand mixer with paddle attachment, beat 1/2 cup butter, 1/2 cup oil, 1 1/2 cups sugar, and 1/2 teaspoon vanilla on medium-high until light and fluffy, about 2 minutes.

Add eggs: Add 4 eggs one at a time, mixing well after each and scraping down bowl as needed.

Combine wet and dry: Alternate adding dry ingredients and mixture of 3/4 cup milk with 1/2 cup lemon juice, beginning and ending with flour. Mix just until combined.

Fold in zest: Gently fold in 2 tablespoons lemon zest. Divide batter evenly among prepared pans.

Bake cake layers: Bake 22–25 minutes until toothpick inserted in center comes out with few moist crumbs. Cool in pans 2–3 minutes, then transfer to wire racks to cool completely.

Make mascarpone frosting: Whip 2 1/2 cups cold cream, 1 1/2 cups powdered sugar, and 2 teaspoons vanilla on high until soft peaks form, 3–4 minutes. Add 16 oz chilled mascarpone and whip until stiff peaks form. Refrigerate until ready to use.

Level cake layers: Use serrated knife to level domed tops if needed.

Assemble first layer: Place first cake layer on serving plate. Pipe dam of frosting around outer edge. Spread half of lemon curd inside dam, then top with layer of mascarpone frosting.

Add second layer: Repeat with second cake layer, remaining curd, and more frosting.

Top with final layer: Add third cake layer on top. Apply thin crumb coat of frosting all over cake and chill 20 minutes.

Final frosting: Apply thick final layer of mascarpone frosting. Create smooth sides or decorative swirls. Garnish with lemon slices or powdered sugar. Refrigerate until serving.