Streusel and filling
1Make the cookie dough first - in a large bowl beat the butter, sugars and salt for a few minutes, until pale and creamy. Beat in the egg yolks and vanilla. Add the flour and beat on low speed or stir just to combine. Take walnut-sized chunks of dough and press into the bottom of muffin tins; alternatively, roll between sheets of parchment 1/4" thick, freeze and then cut into rounds the same size as your muffin tin (I used a small glass rim).
2To make the streusel, in a small bowl, mix the flour, sugar, and salt, then rub in the butter with a fork or your fingertips until well blended. Preheat the oven to 350°F.
3Drop a small spoonful of jam into the middle of each round of dough. Using your fingers or a small spoon, sprinkle 1-1 1/2 Tbsp streusel around edges of each cookie, trying not to get any in the jam.
4Bake for 20 minutes, or until the sides and streusel are golden; cool until warm enough to handle, then use a thin knife to remove the cookies from the pan.