Combine flour, sugar, and salt in a food processor.
Cut butter into 8 slices and add to flour, pulsing several times until mixture resembles oatmeal. Add egg yolk mixed with cream.
Pulse until all ingredients are moistened. Then allow the machine to run a few seconds until the dough starts to stick together. (Add additional drops of cream if the dough seems too dry.)
Press dough into a roll like refrigerated cookie dough. Wrap in plastic and chill 30 minutes.
Spray molds generously with Baker's Secret or something like it. (Baker's Secret is an aerosol spray mix of flour and oil.) If using silicone molds, no greasing is necessary.
Slice your roll of dough into 12 pieces. Using a rolling pin, roll the dough between two pieces of plastic wrap into a circle slightly larger than tart molds. Press into a mold with fingers being careful to keep the thickness consistent. Use a knife to trim off any extra at the top. Freeze for at least 30 minutes, overnight or up to a month. (See video for this step.)
Place frozen tart shells on a cookie sheet. Press a small square of foil down into each crust and mold it to the shape of the crust. See the picture above.
Bake at 400 degrees for 10 minutes. Remove foil and continue to bake until golden brown, usually about 3-4 minutes.