Combine flour, sugar and salt in a large bowl. Cut in butter with pastry blender or two forks until mixture resembles coarse crumbs. Stir in undissolved yeast.: When you work the all purpose flour with the softened unsalted butter , you want a sandy texture, not a paste; this creates a tender dough while still allowing enough structure to form. You will feel small pea sized bits of butter in the flour, and the mixture should look evenly speckled. The aroma will be faintly sweet from the sugar, nothing overpowering yet. This step matters because those buttery pockets melt during baking, leaving tender layers. A common mistake is overworking here and warming the butter too much, which yields a greasy, dense dough, so work quickly and chill hands if needed.
Heat the cream and milk until very warm over stovetop or microwave (120° to 130°F). Add egg yolks, mixing well.: You should feel a gentle warmth when you touch the container, not hot, and you may notice a comforting dairy aroma as the liquids heat. Whisking in the egg yolks creates a silky, homogenous mixture that will help tenderize the dough and add richness. The warmth activates components for a smooth integration but keep it under 130°F to avoid cooking the yolks. If the liquid feels too hot, let it cool slightly; if too cool, it may not meld well. Avoid rapid pouring, which can scramble eggs if temperature control is off.
Add to flour mixture and stir just until dry ingredients are moistened. Spread light film of butter on dough to prevent drying. Cover bowl with plastic wrap and refrigerate overnight or up to 2 days.: At this point the texture should be shaggy and slightly sticky, not a dense ball. The dough will come together as you fold the wet into the dry; resist adding extra flour, because the overnight chill helps firm the dough. Coating the surface with a thin layer of unsalted butter prevents a skin from forming and preserves moisture during slow fermentation. Refrigeration deepens flavor and improves texture, so plan ahead. A frequent error is leaving the dough uncovered or too exposed, which creates a tough outer crust, so check your wrap.
Turn dough out onto a lightly floured surface and knead a few turns to shape into a ball. Roll the dough into a 12 x 16-inch rectangle. Spread with softened butter.: When you knead briefly, the dough should feel springy and slightly tacky, not sticky. Rolling into a rectangle gives you consistent spiral layers; use even pressure and rotate the dough occasionally to maintain shape. The softened unsalted butter spread across the surface should be thin but even, creating a lubricated layer that helps the filling adhere and the spirals seal tightly. If the dough resists rolling or shrinks back, let it rest for a few minutes to relax the gluten; forcing it will result in a tight, tough roll.
Combine brown sugar and cinnamon and sprinkle over butter, followed by the toffee chips. Roll up jellyroll style and pinch seams to seal. Cut roll into 15 equal slices. Arrange cut side up, in a greased 13 x 9-inch pan.: As you sprinkle the brown sugar and ground cinnamon , you will see the surface take on a speckled, caramelized look. The toffee bits add little glints of texture across the layer. When you roll jellyroll style, keep a gentle, consistent tension to form tight spirals; the seam should be pinched to prevent unrolling during rise. Cutting into even portions ensures uniform baking; a sharp knife or dental floss gives a clean cut. A common slip is cutting uneven slices, which can make some rolls under or over baked.
Let rolls rise in a warm place until nearly double, about 1 hour. Bake in preheated 350°F oven for 30 to 35 minutes or until light brown. If rolls are browning too quickly, cover lightly with a piece of foil for the last 5 to 10 minutes of baking.: During the rise you will notice the rolls puff and soften, a visible sign that the yeast is doing its work; the surface will become slightly domed. In the oven, the aroma will deepen to warm caramel and cinnamon, and you will see the edges become golden. The correct visual cue is a uniform light brown top; underbaked dough looks pale and yields a gummy interior. Tent with foil if the tops brown too fast, which is helpful to avoid dark crusts while the center finishes baking. Using an oven thermometer helps prevent temperature surprises.
Remove rolls from oven and let cool a few minutes before removing from pan. Cool 10 minutes and frost.: After baking, the smell is luscious and buttery, and the rolls will relax as steam escapes. Letting them rest makes them easier to transfer without tearing and allows the interior to set slightly, avoiding a runny center. Cooling 10 minutes keeps them warm enough for the glaze to meld but not so hot that the icing melts away entirely. A common error is frosting too soon, which causes a very thin glaze that runs off; wait the recommended time for the best balance of sheen and coverage.
To make caramel icing, melt butter in small saucepan. Add brown sugar and cook, stirring constantly, until mixture just comes to a boil. Remove from heat; stir in milk. Add powdered sugar and vanilla, beating until smooth. Drizzle warm icing over rolls.: As you cook the brown sugar with melted unsalted butter , the mixture will smell richly caramelized and begin to thicken; watch it closely as it can go from perfect to too dark quickly. Once you stir in the milk , the mixture will loosen and steam, and the final addition of powdered sugar gives body and a glossy finish. The warm icing should drizzle easily and form ribbons over the rolls that settle into crevices. If the glaze seems too thick, add small increments of milk until pourable. Avoid boiling too long, which makes a brittle glaze instead of a velvety one.