Bring ingredients to room temp: Butter, eggs, and buttermilk should sit out so they blend smoothly.
Prep pans: Grease and line two 8-inch round pans (or three 6-inch) with parchment. Lightly flour.
Preheat oven: 350°F (175°C).
Pour about 1 cup champagne into a small saucepan.
Simmer over medium heat until reduced to roughly 1/3 cup. Cool completely. You’ll use some in the cake and the rest in the frosting.
In a bowl, whisk 1 1/4 cups all-purpose flour, 1 cup cake flour (or use all all-purpose in a pinch), 2 tsp baking powder, 1/4 tsp baking soda, and 1/2 tsp fine sea salt.
Beat 1/2 cup unsalted butter with 1 1/2 cups granulated sugar on medium-high until pale and fluffy, 2–3 minutes.
Stream in 1/4 cup neutral oil; beat 30 seconds more until silky.
Beat in 3 large eggs, one at a time, mixing 20–30 seconds after each.
Stir in 2 tsp vanilla extract and 1/4 tsp almond extract.
Whisk together 1/2 cup buttermilk and 1/2 cup cooled champagne (use part of the reduction plus plain champagne to reach 1/2 cup if needed).
Add the dry ingredients to the bowl in 3 additions, alternating with the buttermilk-champagne mixture in 2 additions. Start and end with dry.
Mix just until combined. Do not overmix or you’ll lose that fine crumb.
Divide batter evenly between pans and smooth the tops.
Bake 25–30 minutes, until the centers spring back and a tester comes out clean or with a few moist crumbs.
Cool 10 minutes in pans, then turn onto racks and cool completely.
Beat 1 cup unsalted butter until creamy, 1–2 minutes.
Add 3 1/2–4 cups powdered sugar in 2–3 additions, beating on low until incorporated.
Pour in 3–4 tbsp champagne reduction, 1 tsp vanilla, and a pinch of salt.
Beat on medium-high 2 minutes until fluffy. Add 1–3 tbsp heavy cream to loosen if needed.
Taste and adjust salt or champagne reduction for balance.
Level cake layers if needed.
Place one layer on a board; add a generous swipe of frosting.
Top with the second layer. Apply a thin crumb coat; chill 15–20 minutes.
Finish with a thicker coat and decorate with soft swoops or sprinkles. A few sugar pearls give a classy vibe.
