Make cake: Preheat oven to 350°F. Butter an 8-inch springform pan. Wrap pan bottom in heavy-duty foil.
Melt butter and bittersweet chocolate in a large bowl set over a pan of simmering water. Remove bowl, stir to combine and let cool to lukewarm.
Whisk eggs into chocolate mixture until well-combined. Scrape batter into prepared pan.
Place springform pan in a roasting pan; pour hot water to 1 inch deep into roasting pan. Bake until cake is set around edges but wobbly in center, 30 to 35 minutes. Remove foil and let cool on a rack. Cover with plastic and chill for at least 6 hours and up to 3 days.
Make glaze: Put milk chocolate in a bowl. Bring cream just to a boil; pour into bowl. Let stand for 5 minutes; whisk until smooth. Cool to room temperature. Remove cake to a platter; spoon on glaze. Refrigerate for up to 5 hours.
To create spiderweb, spoon canned vanilla frosting into a ziplock bag; snip off a tiny corner. Pipe straight lines from center of cake to outer edges. Connect lines by piping curved lines in between, creating web pattern. Pipe spider or place a candy spider on web.