For the cupcakes: Preheat the oven to 350 degrees F. Line 2 muffin pans with 18 to 19 cupcake liners.
Sift together the flour, sugar, baking powder, and salt in a medium bowl.
Place the strawberries in a food processor and process until pureed. Measure out 1/2 cup of the puree, and reserve the extra for the frosting.
In the bowl of a stand mixer with a paddle attachment, mix the strawberry puree, eggs, Champagne, butter, and vegetable oil on medium speed until incorporated, about 2 minutes. Turn the mixer to medium-low speed, and add the dry ingredients a little bit at a time, making sure to scrape down the sides of the bowl if necessary.
Mix until just combined.
Fill the cupcake liners 3/4 of the way full, and bake until a toothpick inserted in the center comes out clean, 20 minutes. Cool the cupcakes completely.
For the frosting: In the bowl of a stand mixer with a whisk attachment, whip the heavy whipping cream on high speed until soft peaks form, about 4 minutes. (While whipping the cream, place a washcloth over the mixer to prevent splatters on countertop.)
While the cream is whipping, melt the marshmallows in a heat-proof bowl in the microwave on high for 15 seconds. Stir with a rubber spatula, and plop the melted marshmallows into the whipping cream once the soft peaks are reached. Continue to whip until stiff peaks are formed. Turn the speed to medium and add the sugar and 1/4 cup reserved strawberry puree. Whip until all the ingredients are incorporated, about 30 seconds.
To assemble: With a pastry brush, brush the tops of the cooled cupcakes with the Champagne Simple Syrup.
Place the frosting in a piping bag with a tip of your choice, and pipe a large dollop or multiple swirls on the tops of the cupcakes.
In a medium saucepan over medium-high heat, stir the Champagne and sugar together with a wooden spoon until the sugar has completely dissolved. Reduce the heat to medium, and continue to cook until the mixture has reduced to about half and can line the back of the spoon, about 5 minutes.
