Place an oven rack in the center position and preheat the oven to 350°F (175°C). Melt the butter in 20 second bursts, then set aside to cool slightly.
Spray both mini muffin pans generously with baking spray with flour, coating the bottoms and sides of every cup. Set the pans on a rimmed baking sheet.
Let the refrigerated crusts sit at room temperature for about 5 minutes. Unroll one crust on a lightly floured surface, roll it slightly thinner, and cut out 2 1/2-inch circles. Repeat with the second crust until you have 36 circles total.
Gently press one circle into each mini muffin cup, pushing the dough up the sides to form a little cup. Add about 1 tablespoon of water to any empty cups. Slide the pans into the freezer for 10 minutes.
Wash and dry the lemons well. Zest both lemons until you have 1 tablespoon of zest, avoiding the bitter white pith. Roll the zested lemons firmly on the counter, cut them in half, and squeeze the juice through a fine mesh sieve. Measure out 1/4 cup (60 ml).
Add the sugar and lemon zest to a large mixing bowl. Rub them together with your fingertips for about 30 seconds, until the sugar looks damp and smells strongly of lemon. Add the flour, cornmeal, and salt, then whisk until evenly combined.
Pour the melted butter into the sugar mixture and whisk until it looks like wet sand. Add the eggs one at a time, whisking well after each one, then whisk for about 30 seconds more until smooth and a rich yellow color.
Pour in the lemon juice, milk, and vanilla extract. Whisk gently until the filling is smooth and fully combined. Transfer the filling to a measuring cup with a spout.
Take the pans out of the freezer. Pour about 1 tablespoon of filling into each crust, filling each one about three-quarters of the way. Stop before the filling reaches the top edge.
Bake for 18 to 22 minutes, until the filling is set and slightly puffed and the crust edges are light golden brown. The centers should barely jiggle when you gently shake the pan. If your oven only fits one baking sheet at a time, bake in two batches and keep the second pan in the freezer.
Remove the pans from the oven and let them cool for 10 minutes. Run a thin knife gently around the edge of each bite, then lift them out and set them on the wire cooling rack. Let them cool completely, about 30 minutes.
Dust the tops with powdered sugar using the fine mesh sieve right before serving.