For the syrup: Put all the ingredients for the syrup in a saucepan over medium heat and bring to a boil. Once the sugar and honey are dissolved, simmer the syrup for 3 minutes, stirring regularly. Turn off the heat and set the syrup aside to cool. The syrup should be cool when you pour it on the base of the bun.
For the bun base: Place the bun slices in a rectangular pyrex pan, next to each other, without leaving gaps. Slowly pour the cool syrup over the base, allowing time for it to absorb.
For the pastry cream: Place 1 cup of lukewarm milk in a separate bowl. Add the corn flour and mix well to dissolve in the milk. In a large bowl, break the 3 eggs and add the sugar, vanilla extract and mastic powder. Mix all ingredients well. Add the dissolved corn flour to the egg mixture and mix well.
Heat the remaining milk in a saucepan over low heat so that the milk does not burn at the bottom of the saucepan and stir regularly. Immediately before the milk starts to heat up, gently pour the egg mixture into the warm milk, stirring constantly. Slowly bring the cream to a boil without stopping stirring. When the cream starts to boil, add the butter and stir to melt. Turn off the heat and let the cream cool for 10 minutes.
Pour the pastry cream evenly over the bun base, cover with cling film and refrigerate for 2-3 hours.
Spread the whipped cream over the cold cream evenly and sprinkle with the chopped almonds and cinnamon.