Place the oven rack in the center slot and preheat the oven to 375 degrees F.
Put the butter in a small microwave-safe bowl and microwave for 30 seconds or until the butter melts.
Brush the melted butter in each mold of the madeleine pan using a pastry brush.
Put the flour through a fine mesh strainer and lightly dust the flour over the buttered molds.
Take the batter out of the refrigerator. Using a 1-tablespoon measuring spoon and a small rubber spatula, fill each mold in the madeleine pan with 1-tablespoon of the batter. Leave the batter as lumps in the middle of the mold as they will melt to shape in the oven.
Bake in the preheated oven for 10 minutes (convection oven) or until the edges of the madeleines are slightly browned and the top of the madeleines spring back when touched.
Take the pan out of the oven and let it cool for 3 minutes.
Gently lift and remove the madeleines from the molds using a table knife and transfer them onto a cooling rack. Cool the madeleines to slightly warm to the touch, or to room temperature.
Just before serving, dust the tops with confectioner's sugar (optional).