Sift together the cocoa powder, flour, sugar and salt.
Pour the melted butter and vanilla extract into the dry ingredients and fold together until dough forms.
Roll out the dough between two sheets of parchment paper until between 1/4" and 1/2" thick. Transfer onto the tart pan. (The dough may break, that's okay).
Begin pressing the dough into the edges and bottom of the tart pan, patching any gaps with pieces of dough. Leave a 1/2" of dough above the top edge. When dough is firmly pressed into every groove and corner, prick the sides and bottom with a fork.
Cover and chill a minimum of 15 minutes.
Preheat oven to 375°F. When dough is chilled, place tart pan on a baking sheet and bake for 12 minutes then remove from oven.
Separate 1 egg and beat the white with 1 tablespoon of water. Brush the mixture along the warm tart shell right away and return to the oven for 1 minute. Reserve the yolk for later.
Drop oven temperature to 325°F.
Combine the heavy cream, vanilla, and corn syrup over medium heat and bring the mixture to 200°F. The mixture will be steaming and simmering just at the edges of the cream when it hits the right temp.
Pour the cream mixture over the chocolate chips and salt and let stand for 5 minutes.
While the chocolate stands, crack the other two eggs in with the egg yolk and lightly beat.
Begin mixing the chocolate gently, being careful not to beat (will cause bubbles in your custard). When the chocolate is melted and you have a thick chocolate sauce it's ready.
While beating the eggs, drizzle in some chocolate off a fork or spatula. Repeat 4 to 5 times.
Once eggs are tempered, pour them into the chocolate and fold until well combined.
Pour custard into the cooled tart shell and gently lift up one edge, letting it drop gently to lift any bubbles to the surface. Draw a small piece of parchment paper across the surface lightly to pick up bubbles.
Return to oven for 35 to 40 minutes, or until the custard has set to a thick wobble, like jello.*
Remove and transfer tart pan to cooling rack. Let cool to room temperature.
When tart has cooled, carefully set on top of a wide glass and very gently release the tart pan.
Using several long spatulas, slide the tart off of the metal base (only if desired) onto a plate.
Melt the white chocolate in your microwave on 50% power for 20 second intervals, stirring in between until melted.
Using a fork or piping bag, drizzle white chocolate in desired pattern across the top of the tart. For a spiderweb, do many straight lines across the tart, switching directions every few seconds.
For chocolate spiders melt remaining chocolate chips in microwave. Use a toothpick to place a small glob of chocolate onto the tart.
Carefully use the tip of the toothpick to draw the legs out and refine the shape. Let chocolate set, then slice and enjoy!
Store covered in fridge for up to 3 days.
*Or equal amount of dark chocolate instead of chocolate chips. The higher the cocoa %, the more bitter the tart will be. *If baking in a thinner or wider tart shell (I use a 9 inch pan, 2 inches deep), reduce cooking time to 20 mins and check every 5 after. If you see cracks or large bubbles, the tart is becoming overbaked. Remove it immediately.