1. Grease a square or rectangle silicon cake pan
2. Use an electric mixer to beat the butter, sugar and vanilla in a large bowl until pale and creamy. Add the eggs, 1 at a time, beating well after each addition. Add the flour, cocoa powder and buttermilk, in batches, until well combined. Add the vinegar, bicarbonate of soda and food colouring and stir to combine.
3. Spoon mixture into the prepared pan. Use the back of a spoon to smooth the surface. Cook in the slow cooker with a tea towel under the lid ,for 21/2 to 3hrs on high ,keep testing with a skewer until done. Ice when fully cooled.
4. Make the sugar glass. Mix 2 cups water, 1 cup glucose syrup ,white sugar, and cream of tartar in a large saucepan; bring to boil stirring constantly put the heat down to medium.and cook for about an hour ,stirring, The mixture will thicken as water evaporates. , quickly pour onto a metal baking pan. Cool until completely hardened. Break into "shards" using a meat mallet.
5. Make the edible blood. Mix together 1/2 cup corn syrup and cornstarch in a large bowl. Slowly stir in the 1/4 cup of water, adding more if necessary, until the corn syrup mixture has thickened to the consistency of blood. Stir in the red and blue food coloring.
6. Stab the icedcake with a few shards of broken sugar glass. Drizzle on drops of "blood" to complete the effect.