• Preheat your oven to 350°F. Grease two 8-inch round baking pans and set them aside.

First, make the cakes.

• In a large bowl, place the flour blend, xanthan gum, baking powder, baking soda, salt, and lemon zest, and whisk to combine well. Break up any lumps in the lemon zest.
• In the bowl of a stand mixer fitted with the paddle attachment or a large bowl with a hand mixer, place the butter and sugar, and beat on medium-high speed until light and fluffy (about 3 minutes).
• Add the egg, egg whites, and vanilla, and beat on medium-high speed until well-combined.
• Add 3 tablespoons of lemon juice to the milk, and mix to combine.
• Add the dry ingredients and the milk mixture, alternating between the dry ingredients and milk, and beginning and ending with the flour. Beat on medium speed to combine after each addition. The batter will be fluffy but rather thick.
• Divide the cake batter evenly among the two prepared cake pans and spread it into an even layer in each.
• Place in the center of the preheated oven and bake, rotating once, until a toothpick inserted into the center comes out with a few moist crumbs attached (about 25 minutes).
• Remove the cakes from the oven and allow to cool in the pans for 10 minutes before inverting onto wire racks to finish cooling.
• Once the cakes are on the wire rack, while they're still warm, use a pastry brush to paint the bottom of each cake with one of the remaining 2 tablespoons of lemon juice.

Make the (optional) crumb topping.

• While the cakes finish cooling, make the optional crumble topping.
• Place the flour and confectioners’ sugar in a medium-size bowl and whisk to combine well. Add the butter, vanilla and lemon juice and mix to combine. The dough will be thick.
• Place the dough between two small sheets of unbleached parchment paper and roll out into a rectangle about 1/4-inch thick.
• Remove the top sheet of parchment paper and transfer the dough still on the bottom sheet of parchment to a small rimmed baking sheet.
• Place in the center of the preheated oven and bake for 7 minutes, or until just beginning to brown on the edges.
• Remove from the oven and allow to cool on the baking sheet for about 10 minutes before breaking and crumbling into small pieces. Set aside.

Make the lemon cream filling/frosting.

• In the bowl of a stand mixer fitted with the whisk attachment or a large bowl with a hand mixer, place the heavy whipping cream and beat on medium-high speed until stiff peaks form (about 5 minutes).
• Scrape the whipped cream into a separate bowl and place it in the refrigerator to chill.
• In the same mixer bowl, place the cream cheese and butter and beat (with the paddle attachment if using a stand mixer) until light and fluffy (about 3 minutes).
• Add the salt and confectioners’ sugar, and beat on low speed until the sugar is absorbed into the cream cheese-butter mixture.
• Add the lemon juice and lemon zest, turn the mixer up to high speed and continue to beat until the frosting is light and fluffy (about 3 more minutes).
• Remove the whipped cream from the refrigerator and fold it into the cream cheese frosting.

Assemble the cake.

• Place one of the cooled 8-inch cakes, still upside down, in the center of a serving platter.
• Place about one-half of the lemon cream filling on top.
• Spread into an even layer all the way up to the edge of the cake, and top with the other cooled 8-inch cake, still upside down.
• Brush the exposed side of the top cake with lemon juice and cover the whole cake, top and sides, with a very thin layer of the filling.
• Cover with the remaining filling on the top and as much of the sides as you like.
• Sprinkle the top and sides of the cake with the optional crumb topping, pressing the crumbs gently into the frosting/filling to adhere.
• Chill the cake for 10 minutes to make slicing and serving easier.