Whisk together flour, baking powder, and salt in a bowl.

In a larger bowl, cream the butter and sugar until light and slightly fluffy (about 2–3 minutes). Beat in the egg and vanilla.

Gradually add the dry ingredients. If the dough seems too crumbly, add 1 tablespoon of milk at a time until it holds together but isn’t sticky.

Chill the dough for 20–30 minutes; this helps the cookies spread evenly.

Preheat oven to 350°F (175°C). Line two baking sheets with parchment.

Scoop rounded teaspoons of the dough onto sheets about 1 inch apart. Flatten slightly with the bottom of a glass for uniform bites.

Bake for 9–11 minutes until the edges are set and bottoms are light golden. Let cool on the pan for 5 minutes, then transfer to a rack to cool completely.

In a saucepan, heat the milk until just simmering; do not boil.

Whisk egg yolks with half the sugar and cornstarch until smooth and pale.

Gradually temper the hot milk into the yolk mixture, then return everything to the pan.

Cook over medium heat, whisking constantly, until thick and bubbling — about 2 minutes. Remove from heat and whisk in butter and vanilla.

Press a piece of plastic wrap directly on the surface to prevent a skin from forming. Chill thoroughly (at least 1 hour) until set.

Place chopped chocolate in a small bowl. Heat cream until hot but not boiling, pour over chocolate, let sit 1 minute, then stir until smooth. Add corn syrup if you want extra gloss.

Pair cooled cookies by similar size. If pastry cream is fairly stiff, spoon into a piping bag; if soft, you can spoon and sandwich.

Pipe or spoon about 1–2 teaspoons of pastry cream onto the bottom cookie, then sandwich with the top.

Dip the top of each sandwich into the chocolate glaze or use a spoon to spread a glossy layer. Place the cookie bites on parchment and refrigerate for 10–15 minutes to set the glaze.

Serve chilled or at cool room temperature. Store in an airtight container in the fridge for up to 3 days. For longer storage, freeze assembled cookie bites on a tray, then transfer to a sealed container for up to a month. Thaw in the refrigerator before serving.