Preheat the oven to 350°F (175°C). Line a 9×13-inch pan with parchment, leaving overhang for easy lifting. Lightly grease.
In a large bowl, whisk the sugar, melted butter, and oil until glossy. Add eggs and vanilla; whisk until smooth and slightly thick.
In another bowl, whisk flour, baking powder, baking soda, and salt.
Add the dry ingredients to the wet in two additions, alternating with the buttermilk. Stir just until combined. Fold in sprinkles if using.
Spread the batter evenly in the pan. Tap the pan on the counter to release air bubbles.
Bake for 22–28 minutes, until a toothpick comes out clean and the top springs back.
Cool in the pan for 10 minutes, then lift out using the parchment and cool completely on a rack.
Beat the softened butter until creamy, 1–2 minutes.
Add powdered sugar gradually, beating until fluffy.
Beat in vanilla, salt, and 2 tablespoons cream. Add more cream, a teaspoon at a time, until spreadable and smooth.
Chill the cake for 20–30 minutes. Chilling helps you slice cleanly.
Use a long serrated knife to trim edges for neat sides (snack on the scraps). Cut the cake into even rectangles—about 2×3 inches works well. Aim for pairs.
Spread or pipe a generous layer of filling on half the pieces. Top with the matching pieces to make sandwiches. Press gently to set.
Run a small offset spatula around the edges to smooth any filling that peeks out.
Roll the edges in sprinkles, coconut, or mini chips. Or chill and drizzle with melted chocolate.
Chill for 20 minutes to firm up before serving.
