Preheat oven to 350F. Spray two 8" cake pans with nonstick spray. Set aside.
In a small bowl, combine dry ingredients (flour, baking powder, and salt). Set aside.
In the bowl of a stand mixer, combine butter and sugar, beating until light and fluffy. Scrape down the sides of the bowl.
Add half the dry ingredients to the butter mixture, mixing only until just combined. Pour in champagne, mixing to combine, then the remaining dry ingredients. Mix only until incorporated.
In another mixer bowl, whip egg whites with the whisk attachment until they reach stiff peaks.
Add whipped egg whites to batter in three additions, carefully folding in each addition using care not to deflate the egg whites. Continue folding until egg whites are fully incorporated and no white streaks remain.
Divide batter evenly between the two cake pans (see note). Affix cake strips to the outsides of the pans, if desired (see note). Bake cakes 35-40 minutes, or until a toothpick inserted in the center comes out clean.
Allow cakes to cool in pans 10 minutes, then turn out onto wire racks to cool completely.
For Buttercream:
In the bowl of a stand mixer, combine egg whites and sugar. Set over a small saucepan of simmering water, taking care that the water doesn't touch the bottom of the bowl.
Stirring occasionally, warm egg white mixture to 160F. Once the mixture reaches temperature, transfer the bowl to the mixer.
Using the whisk attachment, beat the egg white mixture on high speed until it reaches stiff peaks. The mixture will be white and glossy, and the bowl will be nearly cool to the touch (at least 10 minutes).
Add butter, 1-2 tablespoons at a time, allowing each addition to fully incorporate before adding the next.
Once all butter is incorporated, add strawberry extract (see note) and strawberry puree. Mix until combined.
Frost champagne cakes with buttercream. If desired, leave enough buttercream leftover to pipe a border around the bottom and pipe on top to anchor chocolate-covered strawberries.
For Ganache:
Combine chocolate and cream in a medium heatproof bowl. Microwave on 15-second intervals, stirring after each interval, until ganache is smooth.
While still warm and pourable, pour over the top of the cake, spreading the ganache with an offset spatula and allowing the ganache to drip down the sides, if desired.
For Chocolate-Covered Strawberries:
Line a baking sheet with waxed paper. Set aside.
Melt bittersweet chocolate (or chocolate of your choice) with 1 teaspoon vegetable oil on 15-second intervals in the microwave, stirring after each interval, until mixture is smooth. Allow mixture to cool slightly.
Dip strawberries in chocolate mixture and lay on prepared baking sheet. You can leave them out to set up, or put them in the refrigerator if you'd like them to set up faster.
Once the chocolate is set, melt white chocolate with 1 teaspoon vegetable oil on 15-second intervals in the microwave, stirring after each interval, until mixture is smooth. Allow mixture to cool slightly.
Pour white chocolate mixture into a zip-top bag and cut the corner. Drizzle the strawberries with the white chocolate. Allow white chocolate to set.
Pipe small swirls of buttercream to anchor the strawberries, if desired, and place the chocolate-covered strawberries on top of the cake.Enjoy!
Notes
Note on strawberry extract: I don't think most people have strawberry extract in their kitchen, but I do. I like the added punch of strawberry flavor that it gives baked goods, and the brand I buy doesn't have any artificial flavors (you can find it [here]). If you don't have strawberry extract, you can substitute vanilla extract.Note on using kitchen scale: To ensure I have the same amount of batter in each pan, I use my kitchen scale (link [here]) and weigh the pans. Each of my pans contained about 1lb 14oz of batter (not including the weight of the pans).Note on cake strips: These wet strips affix to the outside of your pans, and help ensure your cake layers bake up with a flatter top, so you cut less cake away when you level the cakes. If you're interested in getting yourself some, you can find them [here]. Note on dark pans and bake time: If you have dark pans, reduce oven heat to 325F. Darker pans bake faster, and the lower temperature will help ensure you don't end up with dry cake.Note on strawberry puree: To make strawberry puree, quarter strawberries and place in the bowl of a food processor. Process until smooth, then strain through a fine mesh sieve to remove the seeds.