Be sure to use almond paste, not marzipan, which has more sugar and is generally used for decorating. It's not the same thing. (According to Nigella Lawson, however, almond paste is called 'marzipan' in the UK.)Brioche, challah, or a close-textured white bread, like pain de mie, all work well. It's best if it's slightly stale as it'll absorb more of the flavorful syrup. And speaking of syrup, be sure to saturate the bread as you want to syrup to go all the way through the slices, not just moisten the top. If you don't drink alcohol, you can flavor the syrup with almond or vanilla flavoring (one that doesn't have alcohol) or ust with orange flower water, adding to taste. Make it a bit stronger than you think since you want it to be present in the finished pastry. Lastly, in the "Do as I say, not as I do" category, sharp-eyed readers will notice I used a silicone baking mat in the pics. Like most of us these days, my head is in the clouds and I prefer to bake Bostock on a parchment paper-lined baking sheet, although you can also bake them directly on a baking sheet if you wish. Silicon tends to "humify" things and while Bostock isn't naturally crispy (they fall between there, and in the "moist" category) the bottoms tend to brown a little when not baked on silicone.
1 1/2 tablespoons, plus 2 tablespoonssugar
1tablespoonrum, kirsch, or amaretto liqueur
a few dropsorange flower water, (optional)
6slices brioche, challah or firm-textured white bread, such as pain de mie, cut in 1/2-inch (2cm) slices
6ounces (170g)almond paste
2largeeggs, at room temperature
a few dropspure almond extract, (optional)
1/2-3/4 cup (40-60g)sliced almonds
In a small saucepan, heat the 1 1/2 tablespoons of sugar with the water, stirring until the sugar is dissolved. Remove from heat and add the liquor and a few drops of orange flower water, if using.
Preheat the oven to 425ºF (220ºC). Line a baking sheet with parchment paper (or leave it unlined) and place the slices of bread on it.
In the bowl of a stand mixer fitted with the paddle attachment, or by hand in a medium bowl with a spatula, beat the almond paste with the remaining 2 tablespoons of sugar, the eggs, and a few drops of almond extract, if using, at medium-high speed until smooth. But avoid whipping it up; mix it just until most of the lumps are removed. (A few little ones won't matter.)
Brush each slice of bread generously with the syrup, enough so it saturates the bread all the way through. Spread each piece of bread with the almond topping then sprinkle sliced almonds over the top.
Bake until the tops of the Bostock are well-browned. I begin checking them at the 6-minute mark but they can take up to 10 to 12 minutes so best to rely on visual clues rather than precise minutes. Remove from the oven and sprinkle with powdered sugar. Serve warm or at room temperature.
Note: The almond topping can be made in advance and refrigerated up to one week. Let come to room temperature before using.