Preheat the oven to 350°F (175°C). Grease and line two 8-inch cake pans.
In a bowl, whisk together flour, baking powder, baking soda, and salt.
In another bowl, cream butter and sugar until light and fluffy. Add eggs one at a time, then stir in vanilla, lemon juice, and zest.
Gradually add the dry ingredients, alternating with the buttermilk, until just combined.
Divide the batter between the pans and bake for 22-25 minutes, or until a toothpick comes out clean. Let cool completely.
In a small saucepan over low heat, stir Limoncello and sugar until sugar dissolves. Cool slightly.
In a chilled bowl, beat mascarpone, heavy cream, powdered sugar, and vanilla until smooth and fluffy.
Brush each cooled cake layer with the Limoncello syrup.
Spread a generous layer of mascarpone frosting between the cakes and over the top.
Whisk together powdered sugar, lemon juice, and Limoncello until smooth. Drizzle over the cake.
Garnish with lemon slices, fresh mint, and a dusting of powdered sugar for an elegant finish.
