Add the cake mix, instant pudding, eggs, melted butter, and champagne to a large bowl. Using an electric mixer, beat until smooth, about 2 minutes.

Cover dough with plastic wrap and place in the refrigerator for 1 hour.

Preheat the oven to 350 degrees (F). Line baking sheets with parchment paper.

Roll the cookie dough into equal-sized balls, about 1-1/2 to 2-inches in diameter. Depending on the size of the balls, you should have 36 - 40 cookies.

Combine equal parts of gold sprinkles with the coarse white sparkling sugar (start with 3 tablespoons each) in a shallow, wide bowl, or dinner plate. Add more sprinkles as needed.

Press the top of each cookie dough ball into the sprinkles and place, sprinkle side up, on the parchment-lined baking sheet. Slightly flatten the tops of the cookies.

Bake cookies for 8 - 10 minutes, rotating cookie sheets halfway through baking. Cookies should be slightly golden brown on the edges.

Cool the cookies on the baking sheet for 5 minutes then transfer to a wire rack to cool completely before filling with the buttercream.

Beat the softened butter on medium-high speed for 2 -3 minutes until creamy. Turn the mixer to low and add the confectioners' sugar, half a cup at a time, beating well after each addition.

Add in the vanilla, salt, and 2 tablespoons champagne. Beat on medium-high for 5 minutes, until light and fluffy. If buttercream appears too thick, add in additional champagne, 1 teaspoon at a time, until desired consistency is reached.

Place the filling into a pastry bag fitted with a large star tip.

Match up two cookies, according to equal sizes as best as possible. Pipe swirls of buttercream on the bottom of one cookie. Gently press the second cookie (bottom side facing the buttercream) into the filling. Repeat with remaining cookie pairs.

Store leftovers in the refrigerator for up to 5 days.