Preheat an oven to 180c (350f) and prepare a 9x9 inch cake pan with parchment paper.
Into a large mixing bowl, add granulated sugar and lemon zest and mix it with a whisk until well combined for about 2 minutes. This helps to release oils from the lemon zest and get a strong flavor.
Add the vanilla extract, kosher salt, sour cream, vegetable oil, melted butter, lemon juice, and limoncello, buttermilk, and mix until well combined.
Add the eggs and mix until well combined.
Add the flour and baking powder and mix until just combined.
Pour the batter into the prepared baking pan and bake in a 180c (350f) preheated oven for about 30-40 minutes or until a toothpick inserted into the cake comes out clean.
Make the mascarpone cream
Into a big mixing bowl, add the heavy cream, vanilla extract, and powdered sugar and mix with an electric hand mixer until stiff peaks form. (don't overbeat the mixture so you don't create butter)
Add the mascarpone cheese (which is at room temperature) and mix it lightly into the whipped cream using a spatula until a creamy mixture forms.
Do not overmix this mixture! Stop just when the mixture is creamy and smooth. If you overmix this, the mixture will curdle and lose its creaminess.
Top the cooled cake with the mascarpone cream and make swirls using an offset spatula.
Use room temperature ingredients. It is important that you use room-temperature ingredients like eggs and butter. This step ensures that your mixture mixes into a homogenous mixture and bakes perfectly.
Measure your flour correctly! When measuring your flour, avoid scooping it with a measuring cup. Instead, stir the flour with a spoon to create as many air pockets as possible, and spoon it into your measuring cup. Finally, level it with the back of a knife.
Use fresh lemon juice. The key to a great lemon cake is the use of fresh, tart lemon juice. Avoid using bottled or concentrated lemon juice, as it can lack the depth of flavor you're looking for.
Use fresh, high-quality ingredients. Opt for plump, juicy lemons and pure vanilla extract for maximum flavor. Invest in good-quality poppy seeds to ensure a satisfying crunch in every bite.
Don’t overmix the batter, you don’t want to develop the gluten in the flour. This step ensures you have a moist cake and not a dense and dry cake.
Scrape down the sides and the bottom of the bowl frequently to ensure that you have a homogeneous mixture that has unmixed parts of the mixture.
Line your baking pan with parchment paper! This step helps the cake to bake evenly, and it makes sure that your loaf cake will release easily from the pan.
Be patient! I know it’s extremely tempting to frost this cake as it is pulled out of the oven, but let the cake cool for at least 1 hour so it will be firmer and won’t crumble up, and prevent the frosting from melting.