Lightly butter the cookie sheet mold (all cavities), then chill the pan 5 minutes so the fat firms up. Dust cavities with rice flour (or AP flour), then tap out the excess. (Rice flour releases best and won’t gum up details.)

Beat butter + powdered sugar on low–medium just until smooth and slightly fluffy, 45–60 sec. Mix in vanilla/almond if using. Whisk AP flour + rice flour/cornstarch + salt; add to the bowl. Mix on low until shaggy; finish by pressing together by hand. Dough should hold when squeezed but not feel greasy.

Divide dough into two discs (3/4 in / 2 cm thick), wrap, and chill 20–30 min (firm yet pliable). For each cavity, pinch off a portion of dough and knead once or twice in your fingers to smooth the surface. Press dough from the center outward into the cavity so it fills the pattern without air pockets. Level the back by scraping a flat edge (bench scraper or offset spatula) across the cavity so the dough is flush with the rim.

Place the filled mold in the fridge 20–30 min (or freezer 10–12 min). Meanwhile, preheat oven to 300–325°F (150–165°C). Use 325°F (165°C) for small/shallower cavities. Use 300°F (150°C) for deep or large cavities. Bake until edges just turn pale gold and surfaces look matte, typically 12–18 min for small cavities or 18–24 min for deeper ones. Shortbread should stay mostly pale, not deeply browned.

Cool the pan on a rack 5–8 min (cookies firm slightly). Invert and tap the pan edge to release the cookies. If any stick, run a thin plastic or wooden tool around the edge; avoid metal on nonstick. Cool completely on a wire rack.